I like the additional detailing mode capability in 2021, but it seams like it does take a lot longer to save a large assembly drawing when the assembly has to be loaded either in lightweight mode or full resolved.  I do not have 2021 and 2020 installed on the same computer, 2020 computer is newer, faster CPU and 64 GB ram (Lenovo P53 and RTX 3000 video). 2021 is on Lenovo P51 with 32B ram and M2200 video.  Six sheets drawing that is 1.7 GB in size in 2021 format while 2020 is 1 GB.  Saving in 2021 is about six times slower than 2020. It using  up to 28 GB of the memory in 2021,