So I just ran an intentionally laggy map (10fps) with autotiles, and it seems the entire display is having to redraw all of the tiles from the very top of the screen, downwards - which would account for the lag. It's only really visible what it is doing when the game is at a low fps though.
AAA games really are getting too big for their own good... First, Just Cause 3 lacks anything much to do beyond destruction missions over a giant expanse of land, and FO4 has too many distracting sidequests from what I hear xD
MV really is much less restrictive to use than Ace. RM is still lacking a few commands though, like checking the direction an event is facing. Here's hoping that will be resolved soon! Same for the memory leak/lag/audio bugs. Still, more freedom than Ace!