I honestly have been wanting to use Rawtherapee forever. Partly because the software is really amazing and it fits a workflow that I have wanted to do where I do the raw adjustments in RT and then move to Gimp for the rest of the “localized and intrusive” adjustments.
By the time I realized I wanted to use RT I already switched to Mac and the software became literally unusably slow. From my research it seemed like RT could not handle the 4K res with the particular hardware choices of apple.
Resolution is the issue. First and foremost like the devs have stated more pixels = more processing time. I tried numerous times to lower the res of the screen but nothing would work.
Ok enough talking turns out lowering screen resolution on a 4k retina iMac only scales the display so you are still technically 4k just “zoomed in”.
I stumbled on a trick where if you are in the Display settings hold the option key and click the scaled radio button you get a list of resolutions. Below the list is a check box “show low resolutions”. click that and every setting of display resolution now has a non 4k version marked (low resolution). Pick one of those and you can still keep your high display size but leave the realm of 4k and boom RawTherapee is snappy and amazing.