After granting GrexIt OAuth privileges, a noticeable delay in email

This will actually happen for any extension which requires many IMAP calls, or any combination of extensions thereof. As a Gmail extension developer myself, it takes a lot of effort to be efficient about your IMAP usage to avoid these kinds of issues.

IIRC, the Gmail tema has said that when most people report of slow Gmail mailboxes, it can usually be traced back to an overload of these extensions.