No. The difference in performance is due to the fact that different code runs if there’s only a single curve and it’s a closed polyline with only 3 or 4 corners. In these cases the surface gets constructed directly from the corner points, resulting in either untrimmed or minimally trimmed surfaces.
It is an unexpected difference though, I’ll have to look into it, see why it’s so much slower.
Update, it’s just the extra work involved in the parallel option. There’s some extra work for every iteration and if you graft the input then there’s some extra work multiplied by the number of items.
So I guess just disabling the parallel computation is the easiest solution here.