Mac OS Sonoma 14.6.1, Lightroom Classic 13.5.1, Macbook Pro 16", M2Max. The GPU is perfectly capable of running two displays with full acceleration (I do it everyday in Final Cut Pro)
However, for Lightroom Classic, enabling secondary display makes the preview very laggy, it drops to 10 or 5 fps from usual realtime update for zoom-in and out, for slider adjustments previews, etc - essentically bringing the user experience back in 1990-ies! Especially pronounced in Develop module (in Loupe seems ok). See attached video for actual demonstration of the issue.
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