I was exploring the nether looking for piglins to trade for ender pearls. After climbing up a hill and almost dying a couple of times I stumbled upon a piglin and began to trade. He gave me blackstone and soulsand. I decided that I could use the blackstone for tools but the soulsand was useless to me then I made the mistake of placing it down and accidentally hit a zombified piglin. After running around a little while and coming across a large magma cube I decided to incase myself in a wall. It was all going swimmingly until I realised I left a corner unpatched and started taking damage from the zombified piglins, so I started spamming them and they all eventually died. Then the magma cube came and I killed it, the medium sized magma cubes spawned and I killed them, then came the small magma cubes which I killed all but one. The final magma cube came and entered through the hole in the wall and started dealing damage every time my red damage cool down ended and despite my decently high health killed me in less than a second.

Me and my friend Bootoyou1234567 decided to experiment with this and we found out that both slime and magma cube have nearly no damage cool down. We tested this by putting ourselves in a small box (slime box 2x2, magma cube box 1x2) with a shield, the result of this was, the magma cube and slime managed to break our shields within about two seconds, damaging so quickly that our shields were stuck on the bottom right of our screens and the loud boasting noise of the shields blocking hundreds of attacks at a time rumbled like thunder. 

Credit gamer tags: Bootoyou1234567, ITeracotaWarior (myself).