The current code for computing tangents is not exactly fast.
This has been a long-standing issue, and recently came up again with T97378.

The main bottleneck is fetching the mesh data, since it's handled through a callback system and each vertex might have its data queried dozens of times.

I've tried a lot of things to optimize mikktspace.c, but unfortunately most weren't that useful:

• Vectorizing SVec3 gives a ~5% speedup, but I'm not sure if the additional ~70 lines of code are worth it
• Keeping an internal copy of the data instead of re-querying all the time helps a lot (~50-60% time reduction), but requires a lot of extra memory (~100 byte per face)
• Going C++ and replacing the internal quicksort with std::sort shows no difference
• Restructuring the entire file to be a header-only library so that the callbacks can be inlined gives ~10% reduction, but is a major change and deviation from the original library

In the end, two simple fixes that actually help remain:

• Don't re-query the number of faces in each loop iteration
• Don't bother looking for identical vertices if there's only one vertex with that hash

With this, time for the test case in T97378 goes from 6.64sec to 4.92sec. It's something I guess.

I feel like completely refactoring this library would not be a bad idea at some point, but for now it does the job...