fan running

Might be dangerous to turn off the fan.

Open Activity Monitor in Utilities, View>All Processes, sort on CPU%, see if anything using too much CPU%.

Open Activity Monitor in Utilities> View>All Processes, Energy tab, sort on Energy Impact.

Have you done a PRAM reset, CMD+Option+p+r...

Reset NVRAM on your Mac - Apple Support

In fact, do 3 in a row, takes a bit of time.

Intel-based Macs: Resetting the System Management Controller (SMC)...

Reset the SMC of your Mac - Apple Support

If @BDAqua's suggestions don't help, then try running the Apple Diagnostics to see if any hardware issues are reported.

What is the exact model of your iMac? You can get this information by clicking the Apple menu and selecting "About This Mac". The model year is very important.