MacBook Air Running super slow after Catalina update
Step one is to open Activity Monitor and check if your drive is IO bound.



Next, once you're sure you're not waiting for storage to do work (by letting Spotlight and iCloud and all the expected programs to finish their work), then look at which programs are taking energy and finally look at the CPU to see what processes are busy.

After a day or two post install and letting the system clean up, it should be substantially faster or you should be able to razor focus on specific apps that are tripping up your Mac. We're seeing excellent performance after upgrading to Mojave and now Catalina across the board on our fleet of Macs. Comment or ask a follow on question if you need more help with pinpointing your blocking process or app.