diff --git a/notes.md b/notes.md
index a1f640fc..6febadc0 100644
--- a/notes.md
+++ b/notes.md
@@ -17,6 +17,7 @@
 # cnc
 #   lathe?
 #   complex process order
+#   double-sided assistance
 # fdm
 #   redo automatic supports using faces instead of deltas
 #   bind process to z ranges or boxed regions
@@ -88,6 +89,7 @@
 * `F` add option to spiral in vs out (optimal tool life) vs mixed (optimal path)
 * `F` add support for tapered ball mills
 * `F` warn when part > stock or cuts go outside bed
+* `F` A-B linked cutting profiles for double-sided milling / part flips
 * `P` crossing open space check point is outside camshell before returning max z
 * `P` detect render message backlog and pause or warn
 
diff --git a/src/kiri/codec.js b/src/kiri/codec.js
index 1b032dd8..774cf53d 100644
--- a/src/kiri/codec.js
+++ b/src/kiri/codec.js
@@ -7,6 +7,8 @@
     if (self.kiri.codec) return;
 
     const BASE = self.base, KIRI = self.kiri, handlers = {};
+    const freeMem = true;
+    const zeroOut = false;
 
     const codec = KIRI.codec = {
         encode: encode,
@@ -123,12 +125,15 @@
     });
 
     KIRI.Slice.prototype.encode = function(state) {
-        return {
+        const rv = {
             type: 'slice',
             z: this.z,
             index: this.index,
             render: encode(this.render, state)
         };
+        // aggressively free memory
+        if (freeMem) this.render = undefined;
+        return rv;
     };
 
     registerDecoder('slice', function(v, state) {
@@ -152,19 +157,28 @@
                     cface: layer.cface,
                     color: layer.color,
                     paths: layer.paths.map(lp => {
-                        return {
+                        const pe = {
                             z: lp.z,
                             index: lp.index,
                             faces: codec.allocFloat32Array(lp.faces)
                         };
+                        if (zeroOut && state.zeros && pe.faces.length) {
+                            state.zeros.push(pe.faces.buffer);
+                        }
+                        return pe;
                     }),
                     cpath: layer.cpath,
                     off: layer.off
                 };
+                if (zeroOut && state.zeros && e.faces.length) {
+                    state.zeros.push(e.faces.buffer);
+                }
                 // console.log('-->',e);
                 return e;
             })
         };
+        // aggressively free memory
+        if (freeMem) this.init();
         return enc;
     };
 
@@ -253,6 +267,10 @@
             array[pos++] = point.z;
         });
 
+        if (zeroOut && state.zeros) {
+            state.zeros.push(array.buffer);
+        }
+
         return array;
     }
 
diff --git a/src/kiri/main.js b/src/kiri/main.js
index 13671428..1c87052b 100644
--- a/src/kiri/main.js
+++ b/src/kiri/main.js
@@ -740,7 +740,7 @@
             totalProgress,
             track = {},
             mode = settings.mode,
-            now = UTIL.time();
+            now = Date.now();
 
         // require topo be sent back from worker for local printing
         settings.synth.sendTopo = false;
@@ -762,12 +762,13 @@
             widget.stats.progress = 0;
             widget.setColor(color.slicing);
             widget.slice(settings, function(sliced, error) {
-                let mark = UTIL.time();
+                let mark = Date.now();
                 // update UI info
                 if (sliced) {
                     // update segment time
-                    if (lastMsg) segtimes[`${segNumber++}_${lastMsg}`] = mark - startTime;
-                    segtimes.total = UTIL.time() - now;
+                    if (lastMsg) {
+                        segtimes[`${segNumber++}_${lastMsg}`] = mark - startTime;
+                    }
                     API.event.emit('slice', getMode());
                 }
                 // on done
@@ -786,6 +787,7 @@
                     SPACE.scene.active();
                     API.event.emit('slice.end', getMode());
                     // print stats
+                    segtimes.total = Date.now() - now;
                     DBUG.log(segtimes);
                     if (callback && typeof callback === 'function') {
                         callback();
@@ -801,8 +803,10 @@
                 }
             }, function(update, msg) {
                 if (msg !== lastMsg) {
-                    let mark = UTIL.time();
-                    if (lastMsg) segtimes[`${segNumber++}_${lastMsg}`] = mark - startTime;
+                    let mark = Date.now();
+                    if (lastMsg) {
+                        segtimes[`${segNumber++}_${lastMsg}`] = mark - startTime;
+                    }
                     lastMsg = msg;
                     startTime = mark;
                 }
@@ -847,21 +851,45 @@
             setOpacity(color.preview_opacity);
         }
 
+        let segtimes = {},
+            segNumber = 0,
+            startTime,
+            lastMsg,
+            now = Date.now();
+
         KIRI.client.prepare(settings, function(progress, message) {
+            if (message !== lastMsg) {
+                let mark = Date.now();
+                if (lastMsg) {
+                    segtimes[`${segNumber++}_${lastMsg}`] = mark - startTime;
+                }
+                lastMsg = message;
+                startTime = mark;
+            }
             API.show.progress(progress, message);
         }, function (output, maxSpeed) {
+            if (lastMsg) {
+                segtimes[`${segNumber++}_${lastMsg}`] = Date.now() - startTime;
+            }
+
             API.show.progress(0);
             if (!isCam) setOpacity(0);
 
             output = KIRI.codec.decode(output);
             if (feature.preview && output) {
+                startTime = Date.now();
                 STACKS.clear();
                 const stack = STACKS.create('print', SPACE.platform.world)
                 output.forEach(layer => {
                     stack.add(layer);
                 });
+                segtimes[`${segNumber}_draw`] = Date.now() - startTime;
             }
 
+            // print stats
+            segtimes.total = Date.now() - now;
+            DBUG.log(segtimes);
+
             API.event.emit('print', pMode);
             API.event.emit('preview.end', pMode);
 
diff --git a/src/kiri/render.js b/src/kiri/render.js
index 05a6ace1..a83fc678 100644
--- a/src/kiri/render.js
+++ b/src/kiri/render.js
@@ -7,6 +7,10 @@
 
     class Render {
         constructor() {
+            this.init();
+        }
+
+        init() {
             this.layers = {};
             this.profiles = {};
             this.stats = {
diff --git a/src/kiri/stacks.js b/src/kiri/stacks.js
index e4a8a28e..11756360 100644
--- a/src/kiri/stacks.js
+++ b/src/kiri/stacks.js
@@ -10,7 +10,8 @@
         emissive = 0x101010,
         metalness = 0,
         roughness = 0.3,
-        newMat = createPhongMaterial;
+        newMat = createPhongMaterial,
+        freeMem = true;
 
     let stacks = {},
         tallest = 0,
@@ -230,6 +231,9 @@
                 ctrl.group.appendAll(mat);
             }
         }
+        if (freeMem) {
+            render.init();
+        }
     };
 
     function createLineMaterial(color, array) {
diff --git a/src/kiri/worker.js b/src/kiri/worker.js
index d5a603cf..a509f853 100644
--- a/src/kiri/worker.js
+++ b/src/kiri/worker.js
@@ -76,7 +76,7 @@ KIRI.worker = {
                 delete cache[data.id];
                 send.data({error: error});
             } else {
-                let slices = widget.slices || [];
+                const slices = widget.slices || [];
                 send.data({send_start: time()});
                 send.data({
                     topo: settings.synth.sendTopo ? widget.topo : null,
@@ -84,7 +84,8 @@ KIRI.worker = {
                     slices: slices.length
                 });
                 slices.forEach(function(slice,index) {
-                    send.data({index: index, slice: slice.encode(state)});
+                    const state = { zeros: [] };
+                    send.data({index: index, slice: slice.encode(state)}, state.zeros);
                 })
                 send.data({send_end: time()});
             }
@@ -125,12 +126,15 @@ KIRI.worker = {
 
         const print = current.print || {};
         const maxSpeed = print.maxSpeed || undefined;
+        const state = { zeros: [] };
+
+        send.data({ progress: 1, message: "transfer" });
 
         send.done({
             done: true,
-            output: KIRI.codec.encode(layers),
+            output: KIRI.codec.encode(layers, state),
             maxSpeed
-        });
+        }, state.zeros);
     },
 
     export: function(data, send) {
@@ -238,20 +242,32 @@ self.onmessage = function(e) {
         run = dispatch[msg.task],
         send = {
             data : function(data,direct) {
+                // if (direct && direct.length) {
+                //     console.log({
+                //         zeros: direct.length,
+                //         sz:direct.map(z => z.byteLength).reduce((a,v) => a+v)
+                //     });
+                // }
                 self.postMessage({
                     seq: msg.seq,
                     task: msg.task,
                     done: false,
                     data: data
-                },direct);
+                }, direct);
             },
             done : function(data,direct) {
+                // if (direct && direct.length) {
+                //     console.log({
+                //         zeros: direct.length,
+                //         sz:direct.map(z => z.byteLength).reduce((a,v) => a+v)
+                //     });
+                // }
                 self.postMessage({
                     seq: msg.seq,
                     task: msg.task,
                     done: true,
                     data: data
-                },direct);
+                }, direct);
             }
         };
 
diff --git a/src/mode/cam/prepare.js b/src/mode/cam/prepare.js
index 13d6ece6..9a3decb1 100644
--- a/src/mode/cam/prepare.js
+++ b/src/mode/cam/prepare.js
@@ -29,12 +29,12 @@
         let point;
         widgets.forEach((widget, index) => {
             point = prepEach(widget, settings, print, point, progress => {
-                update((index * weight + progress * weight) * 0.5);
+                update((index * weight + progress * weight) * 0.5, "prepare");
             });
         });
 
         print.render = KIRI.driver.FDM.prepareRender(print.output, progress => {
-            update(0.5 + progress * 0.5);
+            update(0.5 + progress * 0.5, "render");
         }, {
             thin: true,
             print: 0,
diff --git a/src/mode/fdm/prepare.js b/src/mode/fdm/prepare.js
index ed7e7ec0..1eadde3b 100644
--- a/src/mode/fdm/prepare.js
+++ b/src/mode/fdm/prepare.js
@@ -367,7 +367,7 @@
 
             // notify progress
             layerout.layer = layer++;
-            update((layer / maxLayers) * 0.5);
+            update((layer / maxLayers) * 0.5, "prepare");
 
             // retract after last layer
             if (layer === maxLayers && layerout.length) {
@@ -381,7 +381,7 @@
 
         print.output = output;
         print.render = FDM.prepareRender(output, progress => {
-            update(0.5 + progress * 0.5);
+            update(0.5 + progress * 0.5, "render");
         }, { tools: device.extruders, thin: isThin, flat: isFlat, fdm: true });
         return print.render;
     };