Still waiting for the re-try to finish. I gave it a closed curve this time but didn’t pay attention to start/end position (seam). Is that important? I haven’t tried to understand how it works but find it interesting that @inno’s solution using standard components is considerably faster than your C# solution.
With no sign of progress, it’s difficult to be patient and just wait. There must be a faster way?
Perhaps I’ll get around to trying some of the code in the thread @laurent_delrieu mentioned.
Still waiting… 
P.S. It finally finished. 59 minutes for the C# component, but it looks good (except for the closed ends which will have to be chopped).
celtic knotwork_2022Oct2a.gh (27.4 KB) (slow C# disabled)
P.P.S. It looks like the best way to get speed on this is to do one section and then repeat it.