It is caused by instancing many display lists during recalculation of water model inside the plant.

Reproduction: Add one level to fully built and filled plant

Basically Thermal Evaporation Plant with 10 blocks height creates 51200 display lists

i get a 15sec or so lag when entering area with evap tower as well as when i open the gui. once gui opens the first time i have no other lag. but i am on v300.