May be related to performance of forwarding 2D-mapped events to web content? Or in time to update texture? Or simply in JS and CSS? I'll kick tires on the later to see if I can improve.

Definitely don't spend time on this, you won't be able to improve it. The issue is partially texture latency, partially input latency into web contents, and partially latency from js to our renderer that causes the animation to start a few frames late.

A batch of legacy VR UX bugs have been scrubbed for future work, now summarized here:

http://go/chrome-vr-backlog

UX has approved closing these bugs.  As tasks are scoped, these bugs can be reopened (if relevant), or recreated as needed.