Tried that vertical mirror thing on Sasha-16's default bitmap for the face, its' 4096x4096, it took less than 1 second to flip it vertically using the -1 scale on my aging i7 8700K,.
I've got the feeling that the time needed to invert the bitmap using the scale is only the time to load it: on a 16k HDRi it's immediate as long as the node's preview is off

Note that the MIrror_U and Mirror_Vare used when tiling, as stated in Poser's documentation.
And don't worry: I ignored it till now: it was about time to read the doc because I used to wonder why, too... 

𝒫𝒽𝓎𝓁

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