cf. states diagram https://docs.google.com/a/google.com/drawings/d/1ga3M-3C-3ow0-lbMdqAbPRvB_Jdohl8bxERdMBJi6v8/edit?usp=sharing

After adding trace markers, I can see that pretty much the entire loading time is spent in JS execution. 

Thank you for your investigation.

Can you measure how much time would be saved by preemptive load: (time for keyboard to appear on the first time) - (the second time) ?

FYI: I wrote a change to add state diagram explicitly and will submit it soon.
https://chromium-review.googlesource.com/c/516866/
Hope it helps.

(Design doc: https://docs.google.com/document/d/1f8h6SNIx7szzzVg9d_Wog6eR6ZF1Jo4zHDPfC2s0UOE/edit#heading=h.vqdn9okkt4o5)

The idea of preemptive load is to initiate INITIAL -> LOADING_EXTENSION -> HIDDEN transition whenever (1) the state is INITIAL and (2) keyboard::IsKeybaordEnabled becomes true. 

 Issue 684193  has been merged into this issue.

Here's a video of the new changes: 
https://drive.google.com/file/d/0ByiCx2FXM7kMa20xQ0NxMkt2YWM/view

Oops, looks like it's not committed yet.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d9917f026d85f1881f70d4a74fac74c91579fac6

commit d9917f026d85f1881f70d4a74fac74c91579fac6
Author: Blake O'Hare <blakeo@chromium.org>
Date: Tue Jul 11 05:44:49 2017

Pre-load the keyboard extension when the keyboard controller is activated

This will initialize loading the keyboard extension when the RootWindowController calls ActivateKeyboard.

Bug:  724908 
Change-Id: I3af07338dd3e619f9ae6d85e0f191890d0cd5a67
Reviewed-on: https://chromium-review.googlesource.com/547921
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Keigo Oka <oka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485532}
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/ash/root_window_controller.cc
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/ash/root_window_controller_unittest.cc
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/ash/wm/always_on_top_controller_unittest.cc
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/chrome/browser/ui/ash/keyboard_controller_browsertest.cc
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/ui/keyboard/keyboard_controller.cc
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/ui/keyboard/keyboard_controller.h
[modify] https://crrev.com/d9917f026d85f1881f70d4a74fac74c91579fac6/ui/keyboard/keyboard_layout_manager.cc

Thank you for making the change. Let's see how FirstLoad UMA improves.
https://uma.googleplex.com/p//timeline_v2/?sid=35fdf26fa16dc10197a393e7a4acb187

Will this process be killed when transitioning from tablet to clamshell?

should be closed right?

Yes.