Basically chunk lag may happen if render distance is set to 16 or even near to 16 (for me), the new spherical fog may not be optimized/ may be glitched and cause excessive lag probably due to the new height of the world/ height of the caves.
By putting render distance to something not near to 16 the bug completely disappears
This doesn't have anything to do with the fog, it's just that 16 chunks is a lot to render. If you are rendering less chunks, you're obviously reducing the amount of things to process and thus reducing the lag.