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.