Not sure if this belongs here, so please move if not.
I want to know, what mods do modpacks tend to use to enable them to smoothly run 200+ mods with relatively good FPS and little lag?
I won't go into specifics regarding the net effects, but the options are:
Optifine (by FAR the most popular)
FoamFix (can't say I've ever noticed what it did, other than the obvious progress text that Forge shows me when it deduplicates things)
FastCraft (it was an option in 1.7.10, not sure if it ever moved forward since then)
Phosphor (doesn't try to optimize anything but the lighting engine, in my experience it didn't do a very good job as it created a ton of dark areas wherever elevation changed by even a single block)
(I forgot the name), it optimizes mob AI
Mostly it just depends on your hardware, as after a certain point you can no longer just throw mods at the problem and expect blazing speed.