If you create a new Presert (LUT's) from an external *.cube file the next time you start Photo and select LUT from the Adjustment menu the program is very slow to load the new LUT.
At times it can also result in the program failing and requiring a reboot.
This slow reaction is worse the more new Presets (LUT's) you create.
If you create the new Preset (LUT) using the "Infer LUT" option, and using a reference and sample *.png files the problem does not seem to occur.