Using many skyweave is ok, but dispelling them (converting to simple mirrorweave) drops FPS quite drastically. Funny part is, some blocks working "OK", dealing not so much damage to framerate, but some blocks forces FPS to jump from 77 to 19. Example
Causes heavy lags (drops from ~77 to ~19): dirt, planks (Any, vanilla/mods), awakened supremium gemstone blocks
Causes non-heavy lags (~77 to ~50): crying obsidian