The number of threads to use for a certain function can be difficult. Sometimes more threads than CPU cores makes the function run faster, because on all threads running on the system you get a higher split. Sometimes the overhead introduced with threads makes this inefficient to use a lot of threads.
The plugin functions for threads have another advantage: They yield on the REALbasic thread. So if you call the function Picture.ThreadedTransformMBS inside a REALbasic thread, it will yield time to the main REALbasic thread so you can update GUI:
If you need a plugin function to run faster using multithreading, send me an email.