I tested the same assembly with 2000 parts both in SW2016 and SW2020. Change a part and after that measure the time to rebuild the assembly.  Of course I saved the parts first in SW2020 before testing.

SW2020  25 sec

SW2016  20 sec

From reading the news about 2020 I had the impression we would make a big step forward but it is a big step backwards.  This is 25% longer rebuilding time .