Exactly the same issue in 5.1.0. ray traced shadows hangs editor for 40sec, unusable now. Slowly adding back lights it works for a limited amount. Interestingly turning on transmission for the lights gives better performance.
I am stuck with this, would like to change the version due to alembic hair fix. The project is unusable in 5.1.0 with RT shadow on. Tried a bunch of voodoo and black magic, no solutions so far. For me the shadow batches are the slowest. Not sure what going on, but expected that between 5.0.3 and 5.1.0 there will be no breaks like this. Thanks for the info.