since 5.1 animating in Unity is very slow, it's impossible to make longer animations

we are working in the project more then 2 years, after few updates we had the experience that the animating in unity went slower, but it was corrected in short time by some patch

now after 5.1 it's almost impossible to animate, our animator is going crazy, and as i tried myself while deleting keyframes i was able to count them all before they disappeared (i mean literally one by one watching each of them and summing in my mind)

is there any chance somebody will fix this? or should we just go back to 5.0 ?