Long Lag When attacing a mob

When I use sowrd to attack mobs, after I hit them, they don't get instant damage, instead I have to wait for a long while.

What I expected to happen was...:
Mobs can take damage at the moment I hit them.

What actually happened was...:
Damages  will be taken after a long while.

Steps to Reproduce:
1. I use sword to attack mobs(not only sword, others can also be result in such situation).
2. Then I find that mobs don't get instant damage.
3. I try to hit them for many times, after a long while, mobs can get damage.