Have you blown the dust out of the intakes lately?
What temperatures does this App report? Especially the GPU.
This App can be used to speed up the fans, all my iMacs would be dead without it.
Never ever ever drop fan speeds below the speed that the Mac itself is calling for.
MacFansControl…
https://www.crystalidea.com/macs-fan-control
Have you run Disk Utility's Disk First Aid on all connected drives?
In Disk Utility>View, select Show all Devices, highlight the top left entry.
Run Disk First Aid on all items in the left panel, from top down.
Are you running any VPN, Anti-Virus, or Cleaning apps?
We need to see what all is running, a report from this will not display any personal info...
Using EtreCheck - Apple Community
EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info. Give it Full Disk Access.
https://www.etrecheck.com/
Give etrecheck Full Disk Access before running.
Thanks for Old Toad’s etrecheck instructions…
Slow iMac 2017 - Apple Community