Or considerably faster, as each process can be given own CPU.
If there is such a need to use libraries that the separation cannot be done cleanly, you might be using the languages or libraries wrong and probably get screwed.
For example I recall Java got faster unzipping after C zlib was replaced with native Java. This although it is likely that the C lib was "faster". YMMV, of course.
Personally I would always try to partition the problem into separate processes. Even if it is somewhat slower. For several reasons (architecturally better, more maintainable, easier to test, easier to understand and fix, network transparency, protection from other parts,