Interface code is overhead. Extending the API in C to match exactly what you need and then wrapping the new API in XS will probably give you the "best" results. If you're going for speed you need as few cross-language calls as possible.

As an example, Audio::LADSPA wouldn't be able to generate sound in realtime if you needed a method call for each plugin and frame. That's why it takes buffers of floats and loops through them in C. Looping through half a million scalars a second in perl tends to slow your program down, but C will go through half a million floats in no time. That means it's probably better to write some "higher level" code that does the looping for you, so you only need 1 perl call for a batch of X floats.

Another thing I've noticed is that SWIG's OO interface code is horribly slow (especially when getting/setting properties). That's usually only an issue when the API doesn't really match what you're trying to do, but still it's something to be mindful of.

Cheers,
Joost.