Don’t know exactly how fast it would be, but the BeagleBones have a PRU,
which has two processors, so effectively you have three processors, two
of which (the PRUs) are RISC processors. They only have limited memory
and are really designed for IO processing, but they might be usable.
The original BeagleBoard chips have a DSP built in, which might be
better.
The only real way to find out is to read the TRMs for the chips, and
then try it.
Connecting two(or more) together might help, but you would have to work
out the rate at which you would need to transfer date between them and
see where your bottlenecks would be.
David