The bug

Server-side water updates seem to be roughly 10x slower than in 1.12.
The comparison between 1.12 and 1.13 can be seen in this video by gnembon.

Test to exhibit this behavior:

1. Create a flat world.
2. Execute /fill ~-100 ~ ~ ~100 ~ ~ water
3. Wait for the water to flow completely.
4. Execute /fill ~-100 ~ ~ ~100 ~ ~ air

When the water is removed I recorded a peak at 81 mspt on 1.13 and a peak at (roughly) 8 mspt on 1.12.2.

This makes flushing mob farms unusable: on my survival world, I get peaks > 150 mspt.

The mspt measurment on 1.12 is a bit tricky as there is no way to measure it properly:
I used /forge tps from Forge mod loader to measure it.

Slow down summary