/dev/random uses a lot of system entropy, and so produces only a slow data stream.
/dev/urandom is less secure, and faster, but it's still geared towards smaller chunks of data - it's not meant to provide a continuous stream of high speed random numbers.
You should make a PRNG of your own design, and seed it with something from
/dev/urandom. If you need it a bit more random, seed it periodically - every few MB (or whatever the length of your prng is). Getting 4 bytes (32 bit value) from urandom or random is fast enough that you can do this every 1k of data (reseed your prng every 1k) and get very random results, while going very, very, quickly.