This could be two bugs but I guess the point of this bug is to get a feel of if we are happy with the current performance. It feels less "snappy" than before 

I have a draft CL at https://codereview.chromium.org/2146323004/ that improves the smoothness but at a cost of disabling the window header hiding for the case of many (>10) windows. I would like to get a feel if this is an acceptable trade-off.
This will also likely improve situation with  issue 624608 .

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66

commit d8748a3907d55ade9282b8cf4d0993ac9ee5ab66
Author: varkha <varkha@chromium.org>
Date: Tue Jul 19 16:03:45 2016

[ash-md] Improves smoothness with many windows in overview

Experimentally, most of the animation cost was coming from having window
shapes (used to mask the window header) and from using rounded rectangle
masks.
This CL disables both those performance hogs when there are more than
certain number of windows in the overview mode (controlled via flags).
Default is set to hardcoded 10.
It also makes sure that window controls (minimize / resize / close) are
hidden when in overview mode for custom and panel frames.

New flags are introduced:

--ash-max-previews-to-use-mask=<number>
Maximum number of preview windows in overview mode that can use masks to
hide window headers and use rounded corners. Use -1 to set to unlimited.

--ash-max-previews-to-use-shape=<number>
Maximum number of preview windows in overview mode that can use shapes to
hide window headers. Use -1 to set to unlimited.

BUG= 626851 
BUG= 624608 

Review-Url: https://codereview.chromium.org/2146323004
Cr-Commit-Position: refs/heads/master@{#406286}

[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/ash_switches.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/ash_switches.h
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/wm/overview/scoped_transform_overview_window.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/wm/overview/scoped_transform_overview_window.h
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/wm/overview/window_grid.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/wm/overview/window_selector_item.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/common/wm/overview/window_selector_item.h
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/frame/custom_frame_view_ash.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/wm/panels/panel_frame_view.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/ash/wm/panels/panel_frame_view.h
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/chrome/app/generated_resources.grd
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/chrome/browser/about_flags.cc
[modify] https://crrev.com/d8748a3907d55ade9282b8cf4d0993ac9ee5ab66/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fbfb5afbb61eac2a18e6b210ec4f40162357f612

commit fbfb5afbb61eac2a18e6b210ec4f40162357f612
Author: Valery Arkhangorodsky <varkha@chromium.org>
Date: Tue Jul 26 22:51:04 2016

[ash-md] Improves smoothness with many windows in overview

Experimentally, most of the animation cost was coming from having window
shapes (used to mask the window header) and from using rounded rectangle
masks.
This CL disables both those performance hogs when there are more than
certain number of windows in the overview mode (controlled via flags).
Default is set to hardcoded 10.
It also makes sure that window controls (minimize / resize / close) are
hidden when in overview mode for custom and panel frames.

New flags are introduced:

--ash-max-previews-to-use-mask=<number>
Maximum number of preview windows in overview mode that can use masks to
hide window headers and use rounded corners. Use -1 to set to unlimited.

--ash-max-previews-to-use-shape=<number>
Maximum number of preview windows in overview mode that can use shapes to
hide window headers. Use -1 to set to unlimited.

BUG= 626851 
BUG= 624608 

Review-Url: https://codereview.chromium.org/2146323004
Cr-Commit-Position: refs/heads/master@{#406286}
(cherry picked from commit d8748a3907d55ade9282b8cf4d0993ac9ee5ab66)

Review URL: https://codereview.chromium.org/2184543005 .

Cr-Commit-Position: refs/branch-heads/2785@{#365}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/ash_switches.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/ash_switches.h
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/wm/overview/scoped_transform_overview_window.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/wm/overview/scoped_transform_overview_window.h
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/wm/overview/window_grid.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/wm/overview/window_selector_item.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/common/wm/overview/window_selector_item.h
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/frame/custom_frame_view_ash.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/wm/panels/panel_frame_view.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/ash/wm/panels/panel_frame_view.h
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/chrome/app/generated_resources.grd
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/chrome/browser/about_flags.cc
[modify] https://crrev.com/fbfb5afbb61eac2a18e6b210ec4f40162357f612/tools/metrics/histograms/histograms.xml

Will track this as part of  issue 624608 .