diff --git a/rtiow/renderer/src/renderer.rs b/rtiow/renderer/src/renderer.rs
index c95f357..f73d32f 100644
--- a/rtiow/renderer/src/renderer.rs
+++ b/rtiow/renderer/src/renderer.rs
@@ -501,7 +501,8 @@ fn render_worker(
 }
 
 pub fn render(scene: Scene, output_dir: &Path) -> std::result::Result<(), std::io::Error> {
-    let num_threads = scene.num_threads.unwrap_or_else(num_cpus::get);
+    // Default to half the cores to disable hyperthreading.
+    let num_threads = scene.num_threads.unwrap_or_else(|| num_cpus::get() / 2);
     let (pixel_req_tx, pixel_req_rx) = sync_channel(2 * num_threads);
     let (pixel_resp_tx, pixel_resp_rx) = sync_channel(2 * num_threads);