I can answer that. masterworks has minimal FPS increasers, primarily DFHack item removers and item destroying reducing buildings such as the crematory, along with buildings to accomplish functions that normally reduce fps like the magma wells instead of using pump stacks. Additionally mass creation allowing raw resources to be pushed into large batches of items through everything from Modest Mod inclusions and batch shops, all designed to make the game faster as the need for multiple jobs to accomplish the same task are reduced. Other additions such as colored bricks and usage of blocks to make furniture means reduced mining and decreased rock needs for a sustained fortress.
all and all its not the mods you have that determine FPS. Its primarily the complexity of the fortress, size of embark, age and size of the world, number of civs active in the world, etc. The number of units and the number of jobs currently waiting to be completed also has a huge impact on a map. As I have debugged many maps, the worst offenses are:
setting 1000+ stone to be smoothed,
setting every tree on a map to bee chopped,
digging out multiple (3 floor plus 100x100 plus) large chambers,
opening up the caverns and hell (which never made sense to me they were always open just invisible, suddenly they are a problem when you make them visible),
too many jobs not enough workers,
Fire,
Magma pumped to the surface,
Water dropping any distance,
mist, steam, miasma, any other form of gaseous flow,
overtly spread splatter including raining blood etc.
too many units whether idle or not.
and that last one is the worst problem... generally speaking if 50 units drop your fps from 100 to 50; 100 units will drop your fps to 25; 150 units will drop your fps to 10. and that includes all units, including the pets, attacking armies, random creatures roaming around. And some units cause more havoc to the FPS especially if they cause any of the stuff above that such as fire etc. The only way to compensate is to have a system that works faster, more cpu power to do the path computation faster. everything else, item count etc is minor, you can fix that. I work on a laptop on DF all the time, It doesn't matter how much other stuff I'm doing at any moment. The FPS is going to decline at a steady rate with the increase of units on any given map. i.e. deal with the other stuff the best you can, but it wont matter if you are getting to the 10s by the time you hit 100 units, it wont matter if you have 100 items or 100 million items. Unit FPS reduction is the biggest frame killer once you avoid the other stuff.