I debugged this issue further and can share the following:
vault.enpassdb was initially 850 MB. I exported Enpass data to json and realized that 1 entry had a huge list of password history entries. I manually removed them. The json file size got reduced to 6MB. After I have imported the data from the modified json file, vault.enpassdb is now 400MB.
I have 70 enpassattach files with a total size of 8MB.
I turned off "Use Website icons" and "Check compromised passwords". Having those options turned off, Enpass is more responsive, but still not optimal.
I also get the impression that while Enpass is syncing with WebDav, the main Enpass window is not responsive.