|The library does seem to work, but takes forever to actually output the result.|
I may have been a bit harsh in my comments here. It really depends on the type of problems for which the library is used.
For example, doing calculations with numbers of say 100 or 1000 digits is one thing. But when doing factorial calculations, the number of digits can rapidly grow very large.
As an example, 108654 factorial has half a million digits. Some libraries may calculate this in a moderate amount of time, but struggle with output of the result.
The reason for that is the internal operation may be binary, but the output is required in decimal format.
Anyway, one of my favourite libraries is Michael C. Ring's MAPM
which calculates 108654! in about 9 seconds (on my old laptop). Output of the entire 500002 digit result (to a text file) took 30 milliseconds.
Some of the other libraries may
be somewhat faster in the calculation, but could take minutes or perhaps hours to produce the output.
So - choice of library may depend on the goals.