Also have had this issue since 6.0, tried all the suggestions to no avail. I will note however, that the installing the trial version of immerse spatial audio did seem to help until 6.01 as others have also mentioned, now it doesn't seem to make any difference whether it's on and running or not.

Another user mentioned adjusting power settings (back a couple pages), and adjusting ffxiv to high priority in task manager, this seemed to help a bit? So may be worth trying but like everybody else, nothing seems to fix it completely. At this point I've just gotten used to it, cheers to all the people actively trying to troubleshoot as it seems we'll be on our own for a while :/