Hello,
I've not heard anything about "terribly long" sync times, long ago or recently.
One possible cause is the excessively large "messages per page" setting (either app-global, under Mail, receiving; or account-specific, under options and folders -> account options). This is how many recent messages the app keeps up to date when checking mail, every time.
The default is 25, a much larger value will take much longer.
And then of course there are debug logs. Please see below (in my signature).