The BMesh to Mesh conversion does some checks to the UV helper
attributes like selection to avoid copying them to the mesh if they
don't contain any meaningful data. However, it does this by looping
over all faces for every UV map, not in parallel, so it takes up a large
portion of the total time in the conversion. This commit moves that
to the existing similar checks.
On a 1 million face mesh with 3 UV maps, for me this improved the
conversion runtime by 75%, from 174ms to 99ms. Before the serial
loops took 88ms out of the total. Combining them with the existing
loop over faces only increased its runtime from 29 to 40ms.
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