Since updating my server to 20w17a, my enderman XP farm has been giving very very low rates, as in 5-10 endermen spawning every minute. before this, it has been about 80-100. Although these are rough estimates, it is clear to see, that the endermen are spawning less frequently. I've been reading the patch notes, and it says nothing about endermen spawn rate being reduced in the end, so I do believe this is a bug. I've changed nothing, and compared 2 identical builds in a creative world to test, and the 20w17a is significantly slower.

To reproduce
1. build an enderman farm of any design
2. compare spawn rates of enderman between 20w16a and below and 20w17a

although pictures arent much good, i have a picture of 20w16a and 17a to compare