Blender Developer Jeroen Bakker authored a patch moving the current UV/image editor drawing to the draw manager.
This patch allows for a massive increase in performance when previewing large image files, as well as correctly displaying alpha and emission colors.
Twitter user @TheLapisSea demonstrates the leap in performance
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— L?????apisSea ? (@TheLapisSea) September 12, 2020
This commit just landed in the latest Blender 2.91 experimental build, which you can download to test out the improvements right away!