While it's interesting that we have a new boss which is the rat king, I find that is one problem.
his death animation is too long which takes around 3-4 seconds of a rift. I find that it could influence a greater rift clear because of how a player killed the rg with around 4-5 seconds to spare results in a instant clear. But with Hamelin killed around 4-5 seconds, i find that players who kill that rg by that time will not finish it because of his long death animation.
To give you a reference, here's one example of Hamelin's long death animation:
death starts around 2:32 and takes around 3 seconds.
Solution, devs need to make his death animation instantly. It's really not necessary to have a long death animation that potentially could make players not finish a grift in time due to a discrepancy like this