diff --git a/crates/coremem/src/driver.rs b/crates/coremem/src/driver.rs
index e0e72da..92ab114 100644
--- a/crates/coremem/src/driver.rs
+++ b/crates/coremem/src/driver.rs
@@ -277,7 +277,7 @@ where
 
         trace!("step begin");
         self.diag.instrument_step(steps_this_time as u64, || {
-            self.state.step_multiple(steps_this_time, &stim);
+            self.state.step_multiple(steps_this_time, &*stim);
         });
         trace!("step end");
 
@@ -451,9 +451,11 @@ struct StimAccess<R, T> {
     /// A.K.A. "can i safely do a blocking recv on response_channel".
     outstanding: Cell<bool>,
     /// data sent from worker thread back to the Driver side.
+    /// XXX: Boxing isn't necessary, but doing so means much less memcopy'ing over the channel
+    /// (just one pointer, instead of N^3 bytes). better for perf.
     response_channel: (
-        SyncSender<(SimMeta<f32>, u64, RenderedStimulus<R>)>,
-        Receiver<(SimMeta<f32>, u64, RenderedStimulus<R>)>,
+        SyncSender<(SimMeta<f32>, u64, Box<RenderedStimulus<R>>)>,
+        Receiver<(SimMeta<f32>, u64, Box<RenderedStimulus<R>>)>,
     ),
     worker: ThreadPool,
 }
@@ -483,7 +485,7 @@ impl<R, T> StimAccess<R, T> {
     /// waits for an outstanding job (if any).
     /// if the response matches the request, return the response,
     /// else discard the response.
-    fn maybe_wait_for_job(&self, meta: SimMeta<f32>, step: u64) -> Option<RenderedStimulus<R>> {
+    fn maybe_wait_for_job(&self, meta: SimMeta<f32>, step: u64) -> Option<Box<RenderedStimulus<R>>> {
         if !self.outstanding.get() {
             return None;
         }
@@ -501,7 +503,7 @@ impl<R, T> StimAccess<R, T> {
 }
 
 impl<R: Real, T: DriverStimulus<R> + Send + 'static> StimAccess<R, T> {
-    fn get_for(&self, meta: SimMeta<f32>, step: u64) -> RenderedStimulus<R> {
+    fn get_for(&self, meta: SimMeta<f32>, step: u64) -> Box<RenderedStimulus<R>> {
         // either claim the outstanding job (if it exists and matches)...
         self.maybe_wait_for_job(meta, step).unwrap_or_else(|| {
             // or start a job and wait for it to complete inline
@@ -525,13 +527,13 @@ impl<R: Real, T: DriverStimulus<R> + Send + 'static> StimAccess<R, T> {
             let rendered = diag.instrument_stimuli(|| {
                 let stim = stim.lock().unwrap();
                 let opt = stim.optimized_for(meta, step);
-                opt.as_ref().rendered(
+                Box::new(opt.as_ref().rendered(
                     meta.time_step().cast(),
                     // TODO: convert this to an integer
                     meta.time_step().cast::<R>() * R::from_primitive(step),
                     meta.feature_size().cast(),
                     meta.dim()
-                ).into_owned()
+                ).into_owned())
                 //^ this 'into_owned' ought to be a no-op.
                 //^ it would only ever be borrowed if we accidentally called `rendered` twice.
             });