On some of my creative worlds, I run a command (visible below) that applies Regeneration I to a mob so that they can passively regenerate health should they take damage.

/effect give <mob ID> minecraft:regeneration 1000000 0 true

The command gives the mob the effect for 1,000,000 seconds (~11 days) so I don't have to continuously re-apply the effect. Whenever I apply this effect on a mob, the mob slows to a snail-like crawl and can barely move - this was tested on both an iron golem and wolf.

Edit: Furthermore, removing the effect does not resolve the issue. Applying this command permanently breaks the mob's AI.

It would be preferable to give players the ability to apply these effects indefinitely, like modifying a data tag.