Same here! Paper2d project and since 4.8 it takes 5 sec to go to 95% and then it gets stuck there for 10 minutes until it finally loads up. Mind you that i have zero 3d stuff (like complex materials,lighting etc) that could warrant such a delay.
And another weird thing that i am positive that is related to above issue is this:
When it finally loads up and i press play(standalone window or simulate) preview starts in less than a second. Now if i change ANYTHING on an old BP that has references all over my game (by anything i mean even to uncheck a setting or to just edit a comment) then it takes 5 minutes(!!!) to start preview .If i create a new blueprint with no references it lets me edit crap out of it without affecting performance whatsoever .
It seems that there is a big bug hidden in there that re-calculates everything whenever we change something. This makes active development almost impossible.