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Yes you are correct. I just ran Instruments on the Mac Pro. The hottest spot is CGColorTransformConvertUsingCMSConverter in macOS 10.15.3.

The color conversion is necessary IMO, without proper conversion, the display will be over-saturated, just like switching a 10bit display to 8bit RGB.

Can you cache the converted thumbnail bitmaps so that the big window content doesn't have to be updated as a whole?