I've enabled WebRender a couple of days ago (flipping gfx.webrender.enabled). Opening a Private Mode window on macOS takes almost a couple of seconds, while the same operation is normally instantaneous. Not sure if it's the Mac specs (it's quite old, iMac 27-inch 2010). Model ID: iMac11,3 CPU: Intel Core i5 2.8 GHz RAM: 12 GB Graphics: ATI Radeon HD 575 1GB

Note that it's not a recent regression: I've tried builds up to Feb 1, and they already have the issue. Not sure if the lack of animation making it look worse, but it seems slower. Same version of Nightly, different profiles https://youtu.be/kAPkF7t_Fi8

Is opening a new private window slower than opening a new regular (non-private) window? We know that opening new windows is slow in general with webrender because of the extra overhead.

(In reply to Kartikaya Gupta (email:kats@mozilla.com) from comment #2) > Is opening a new private window slower than opening a new regular > (non-private) window? We know that opening new windows is slow in general > with webrender because of the extra overhead. Thanks. New windows are indeed as slow, didn't think of checking (I open PM very often, new windows not so much). I guess it should be duped against bug 1423671.