For some reason the more Skyproc Patchers I install, the more slow opening perk menu becomes up to slideshow. Using ASIS, Automatic Variants, Dual Sheath and SkyRe Reproccer. If I dont use patchers, its fine (but still abit slow for a moment).

What can it be?

My modlist is big, about 450 mods total.

You provided your own answer.

I thought the amount of mods were capped at 255?

for the NMM it is, but technically you CAN install more than that, just not recomended as it can (and useally will) cause severe issues as seen here.

OP, HOW in the world do you have over 400 mods O.o