I have also noticed PL4 becoming slow. It takes 10…20 seconds just for “Full Preview in Progress” after defining or even slightly altering a local adjustment mask or its adjustments. I have not found a solution to speed up.
It also takes an awfully long time to load NEF images to PL.
WIN10 Home 64,
Core i7-4770, 3.40GHz,
NVIDIA GeForce GTX 760,