Sometimes on my Lenovo C630 running Nightly (on Windows 10) I get quite bad tab switching lags.

https://perfht.ml/2RQBHn0 is a profile showing when this most recently happened (just now)

Here's the relevant part of the profile: https://perfht.ml/2ROPlqT

It looks like the main thread is waiting on Shmem'ing, and flushing paint threads.

Hey overholt, can you post another profile, this time with PaintWorker threads as well?

(In reply to Mike Conley (:mconley) (:⚙️) from comment #1)

Hey overholt, can you post another profile, this time with PaintWorker threads as well?

https://perfht.ml/2RR97lA

Here's the relevant part of the profile:

https://perfht.ml/2RO6Jw3

In this example, it looks less like the paint threads taking a long time, and more that the compositor was bogged down.

I think at this point, it's worth pointing someone from Graphics at this.

It looks like the driver is doing a bunch of CPU work inside CContext::TID3D11DeviceContext_UpdateSubresource_, i.e. texture upload. Unfortunately we don't have symbols for qcdx11arm64um850.dll. Texture upload should be a simple memcpy so this is a bit surprising.

Once WebRender runs on arm64, we should check whether it improves the situation here.