Had a similar issue/problem with my Late 2012 iMac 27. Ran fine on High Sierra (the last 32-bit version of MacOS X), but got really slow running Mavericks (the first 64-bit version).
I fixed mine by running the system from an external USB3 SSD (Samsung T5). It now boots from cold in under two minutes. Starting something like Word or Excel, or Lightroom takes just a few seconds. Really night and day difference from running off the internal spinning disc.
I have some 32-bit apps, so am still running Mavericks because even though it’s 64-bit, it will still run 32-bit apps. Catalina won’t do this apparently.
If you have a TimeMachine back up of your system from a time before you ‘upgraded’ to Catalina, then reverting to an earlier version of MacOS is easy. However, you will need to manually backup any recent documents, photos and so on before restoring to an earlier TimeMachine backup.
If none of this is making any sense, you unfortunately have a bit of a learning curve ahead of you. But keep us in the loop, we may still be able to help.