The cooldown between hits or the attack speed is not correct.

For example, you hit an iron golem three times in about a second with a sword, then you will notice that the durability of the sword will go down by 3, but the iron golem only takes the damage one time.

Another example, you crit-hit an iron golem extremely quick(something like 3-6 times a second while tapping on the space bar), you will find that there will be more than one spark (or whatever you call the things that appear when critical-hitting an entity) coming out, and the durability will be going down very quickly, but the entity only takes the damage one time.

So basically, when you hit entities really fast, it only takes the damage one time every few swings, but your durability will drop down every time you swing at them.

I also check the items (blocks and stuff), but it seems like they also have no cooldown between hits.