Alright! I'm having a VERY odd issue (least it's new to me, after hundreds of games).
Simplified, there's two 'symptoms' of this issue I encountered.
#1 is Brick Ovens. When I mouse over them, they lag for incredible amounts of time before finally being selected. I.E. anytime I move my cursor in range that would 'select them' - it lags for 3-30 seconds (gets worse as time goes on), before showing the menu for the oven.
#2 when building constructions (Floors, walls, stairs, anything from the construction menu) - I noticed a GIGANTIC leap in lag that correlates to the oven lag. Each block placed takes 1-12 seconds to be 'placed' - so when trying to place 2000+ wide areas (say flooring a large room) - it can freeze my game for 40+ minutes in real life before unfreezing.
This seems to be related to open spaces. Start of a map? absolutely flawless speed. Dig out maybe 2-3 floors of the map (I used to dig out 50+ floors without issue on a 4x4 embark) - it lags, and grows worse and worse the more 'open space' there is. Even if that space is exposed to the surface (outside), or underground (inside) - means diddly squat.
Only seeing this on dwarves atm. Nothing 'special' about the embark - had this issue across 4 maps, ranging from glaciers, tundras, swamps, forests, etc. Good, evil, neutral, savage, tame... everything. Same results.
Version:V4J
Tileset: Masterwork - Obsidian walls
Other information:
~ had the issue in an old install of v4j - reinstalled fresh. Same results.
~ did not install over old install or anything similar. No raw editing, nothing.