Google Chrome tab switching etc. very slow - unusual fix

(I posted this to the Google Product Forums but never got any help)
https://productforums.google.com/forum/#!topic/chrome/rY7tI_eAshA;context-place=mydiscussions

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I have Version 61.0.3163.100 (Official Build) (64-bit) of Chrome installed on Windows 10.

I have four active profiles running (i.e. separate signed-in Google accounts - call them "main", "work", "personal", "darkwebz").

One profile - my oldest and primary account - is the one with the following issue:

Creating a new tab, switching tabs, or creating a new window, takes about 2 to 3 seconds. I have disabled all of my extensions and rebooted - even signed out the profile and rebooted - every time I sign in it brings back the delay.

Interestingly, while this delay occurs with the primary profile, the rest churn along quite happily - open / close / switch tabs is near instantaneous. This is all happening on the same machine with everything running; so it's not a specific CPU problem or resource issue - at least, I can't see how. As a test, I opened 40 tabs on the primary, and 80 on another profile, and the primary is slow, the alternative is lightning fast. That being said, page navigating within a tab is not affected - only new tabs, tab switching, and general navigating around tabs.

After exhaustively trying to disable every extension, refresh, reboot, sign out, sign in, etc. I happened upon one little tidbit that seemed to affect everything - the moment the Chrome theme loaded up, it all went to pot.

I then disabled the Theme and presto! Everything is normal and fast!

So, when in doubt, try reviewing the Google Chrome theme ...