When a mob jumps onto a block, it jumps very slowly, and looks too smooth. This also occurs when you hit a mob, and it bounces back very slowly, opposed to bouncing back fast like in versions before.

The only mob that seems unaffected at all by this bug is a beached squid, which, when on land, still gets knocked back quickly, and not slow like the other mobs. This leads me to believe that it has nothing to do with the game's code, but rather only the mobs' code.

It should be noticeable, but many people do not notice it as easily. A great way to notice this bug is by comparing any mob's jump/knockback speed to that of a squid, or go back to 1.7.10, test the mob speed there, and come back to 1.8. You will most definately notice the difference.

This happens regardless of mob gamerules like /doMobSpawning false. This issue is unrelated to MC-58120.