Mobs barely move or not at all.

I tamed a horse and began to ride it around. After a good while of riding it, it started to slow down. I got down and could move fine, but when I got back on it, it was slow and so were all other mobs. I exited the game for a while, but when I returned, even though I wasn't riding the horse anymore, all mods were slowed. Now it seems like either any mob I encounter is either slow or not moving at all. But I can move fine.

What I expected to happen was...:
That I could just keep riding without a problem.

What actually happened was...:
The horse suddenly began to stop

Steps to Reproduce:
1. I traveled over 4000 blocks from my bed.
2. I tamed a horse and rode it back to my bed.
3. More than 2/3 back, my horse began to slow.