The bug

Previously, a server computed for mobspawning from y=1 to the highest subchunk in which there is an opaque block. (From full version 1.13 not every subchunk but every block up to the highest opaque block in this chunk.) But in the current version 1.14.2 (and also until 1.14.3 Pre Release 2) the server calculates up to the highest block in general, even if it is glass.
This slows down many mob farms, as, for example, the afk position is often built out of glass above the mob farm. That sounds like an unimportant bug, but for technical players this is very significant.

How to reproduce

1. Create a superflat world with preset minecraft:air;minecraft:plains;
2. Run /fill ~ 0 ~ ~24 0 ~24 stone
3. Fly up to an appropiate distance to the platform that allows mob spawning
   → Mobs spawn extremely fast
4. Run /fill ~ 255 ~ ~24 255 ~24 glass while being on the same x, z coordinates you were while executing the first command
5. Run /kill @e[type=!minecraft:player]
   → Mob spawn rate is reduced significantly