You can lower idle RPM to ~950rpm on an iMac with a trick:
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Install smcFanControl, then open Terminal and copy and paste the following:
Afterwards, launch (or relaunch) MacsFanControl (it's important to launch or relaunch it after you've modified SMC, otherwise it won't be able to detect the new minimum), set your fan control to manual and adjust the values for start and maximum temperature to your liking. There's a noticeable difference between 1,200 and 950rpm, and it made a huge difference for me.