You can stop synchronizing/indexing each time you switch to the IDEA and it’s quite useful when dealing with big projects and outside build process which triggers indexing.
Just disable checkbox System Settings -> Synchronize files on frame or editor tab activation.
Make sure you run indexing manually to update hints when needed through Synchronize menu (Cmd-Alt-Y on Mac) or File -> Reload All from Disk to run it manually for IDEA 2020 running under Linux
