Delay in life leech and life regen...

I noticed a delay, is it on purpose? Or is it dysync? For example if monster hits me hard my life goes down but for some reason there’s like a delay in regaining my life. Then poof goes up quick...

No its just your leech need to build up enough instances before its noticeable, happens because you do have low damage, low leech and/or hit slow and most noticeably hit just one target.

The leech mechanic is as such that it will reset all leech instances each time you hit max life.
Leech instances are added up and they are caped at 20% of max life/s unless modified.

Default leech instance is 2% of max life and will last until max life is reached or total damage is leeched as life. There is plenty of mods that affect leech tho.

Hitting one enemy for example grant you 2% life recovery/s, you need to hit it 10 time to get your leech rate caped unless you have leech rate modifier. 100% life leech rate will mean you need to hit it just 5 time for example.

Hitting 5 targets will need just 2 attacks and with 100% leech rate just 1 attack to hit 20%.
Each time you hit max life all instances are reset and it need to build up again.

Slayer got his leech mod that ignore the max life state and will keep his leech instances until they expire.
Vaal pact double life/s leech cap and life leech rate. Some notable rise the cap for ~3% and VP double all of them after, life recovery mod grant bonus to all and is not restricted with max life/s leech cap but do not contribute in building up to leech cap ether.

Thanks that helped ^.^