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I don’t think so. The image isn’t rendered twice. It’s just mirrored.
It gets copied into the picture viewer. So yeah, there is some overhead, but it’s not so big and it gets copied only once in a while.
On the other hand, not refresh the material preview when the IR view is active ? wouldn’t that relieve the processor ? when the IR view is active the material preview is secondary … for me of course, maybe not for other.
For me that would be taking a step back to be honest. I like to work with both previews 
you can stop/pause IR, set halt pass limit, set preferences to not show VFB in PV, use viewport preview… you can choose differently to optimize your workflow - is just a click away
ps
PV is IMO very limited by how Maxon chose to design it (& sometimes a bug appears that clamps every result to 8 bpc - why i never use PV)
also the lack of color management in Cinema… ay, ay, ay…
