Starting with SOLIDWORKS 2020 there is a new feature called "Detailing Mode" that's specific to drawings and can be very helpful for making simple edits to larger drawings. However, there are some scenarios that can cause the save operation to take much longer than expected in 2020 due to this new feature saving that additional detail mode drawing data.
Starting with 2020sp4, SOLIDWORKS has added a system option that can allow the user to disable this functionality during the save operation which can improve save times for drawings. This bug only impacts 2020 and 2021 installations.
Open SOLIDWORKS and go to Tools>Options>Drawings>Performance, and uncheck the option "Include detailing mode data when saving". Close SOLIDWORKS and try the save operation again and it should be much faster.
For more details, search for SPR1181559 in the SOLIDWORKS Customer Portal.