STRs: 1. Open https://jsfiddle.net/jib1/pz5pynyf/ and share camera + mic 2. Hover over tab to reveal process id. 3. Open OSX Instruments Time Profiler and select process id. 4. Record 10 seconds then stop. 5. In profiler's search box drop-down, check "Involves Symbol" 6. Enter "DeliverFrame" (without quotes) 7. Expand until you see MediaEngineRemoteVideoSource::DeliverFrame, then 2 more. Expected result: 33.00 ms 1.2% 1.00 ms mozilla::MediaEngineRemoteVideoSource::DeliverFrame(unsigned char*, mozilla::c... 30.00 ms 1.0% 0 s mozilla::layers::SharedPlanarYCbCrImage::CopyData(mozilla::layers::PlanarYCb... Actual result: 646.00 ms 18.5% 0 s mozilla::MediaEngineRemoteVideoSource::DeliverFrame(unsigned char*, mozilla::c... 319.00 ms 9.1% 0 s webrtc::I420Buffer::CropAndScaleFrom(webrtc::VideoFrameBuffer const&, int, i... A 20x slowdown. Regression range: 23:06.53 INFO: Last good revision: bbac8c59d58c50fec337b4832ad8ab28bc507bee 23:06.53 INFO: First bad revision: f2ce61d1a59297b7ad3c91496196fa3a0b776aa7 23:06.53 INFO: Pushlog: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=bbac8c59d58c50fec337b4832ad8ab28bc507bee&tochange=f2ce61d1a59297b7ad3c91496196fa3a0b776aa7 Points to Bug 1393119 - Add webrtc gn build config; r=dminor There's already another bug 1438554 pointing to fallout in this area.

Bug 1439529 seems related to me. You sure this is not a dupe? The regressor would then be bug 1433552.

The patch for Bug 1436959 should also fix Bug 1438554. It hasn't occurred since Bug 1436959 landed, but it was pretty low frequency before, so I'm not going to call it fixed just yet.