Title says it.

Started seemingly randomly, as it was fine before. I had been using questie for about 4 days (levels 18-25), and now when I mouseover an npc, the game skips a frame or two (with i7-4790k, no lua error, probably just some extremelly unnecessary computation).

Tried resetting config, NUKEing, deleting WTF, recopying addon files Nothing fixed it.

It's not Questie... it's the WoW client. This tooltip issue has existed for a long time. Check out some of the old Blizzard Forum posts about it.

Aha ok, is there any fix then? Does reinstall help?

The only thing I came across to mitigate the issue is to keep the number of mods that modifies the tooltip to a min and empty your WDB cache when it starts to become noticeable.

After turning in the quest Items of some consequence, the lag went away. So I suppose it's the part that required Silk Cloth as so many mobs can drop it?