Help needed. Lightroom very slow, uses >450% CPU and makes iMac fan run significantly

If the CPU is often at 450%, then it isn’t purely a heat problem, or the heat is because the CPU is very busy. In Lightroom Classic there are various background operations that can do chores when nothing is happening, do you know if it makes any difference if these are disabled? Some include:

• Replace Embedded Previews with Standard Previews During Idle Time (in Preferences / General) 
• Generate Previews in Parallel (in Preferences / Performance) 
• Address Lookup and Face Detection (Identity Plate) 

These background operations are usually no problem on newer Apple Silicon Macs with lots of efficient (cool running) CPU cores to spare, but they are more likely to strain an 6-year-old iMac that has only four Intel CPU cores.

Fans run because there is heat building up inside the case. Heat must be dissipated, so you need to check the entire cooling system to make sure it is working properly, including fans and vents. Vaccuum out all exterior vents. Consider getting a laptop cooling device.

The first thing I would check on your iMac is to see if the cooling ducts are clogged with dust or animal hair. You might also run the Mac Activity Monitor to see if other apps are contributing to the workload on the processor.  I'm a MacBook user, but I have several friends on iMacs of that vintage and they don't have that issue with the latest version of Lightroom Classic.