So I was playing 19w07a, and noticed a fox hunting fish. However, they were extremely slow in the water, and the hunting looked extremely awkward since they were moving at a snail's pace.

I remembered this old bug (MC-48616) that caused mobs to move slowly in water, and started to research this, and so I tested it in different versions.

14w05b (before the bug) : Zombie: 9.5 seconds to swim to the player. (14w06a actually introduced it)

15w45a ("fix version"): Zombie: 18 seconds to swim to the player. That's about TWO TIMES slower than before.

Latest version: Zombie/Creeper: about ~15 seconds to the player in water (Zombies now sink to the bottom so I tried to test it with Creepers too)

But Foxes are the easiest mob proof that this bug still exists. I also think fish are surprisingly sluggish for a water mob, so they might be affected by this bug as well.

How to reproduce:

Make a mob chase another/player mob in water, for example:

Zombie chase a player or a villager

Wolf chase a sheep

Drowned chase a player of villager, but in 1-block high water

Notice how slowly they move.

You can compare this to any version before 14w05b to see how much faster the mobs were in water.