Raspberry pi performance should not be the issue, since Bitfury made miners with hundreds of gigahashes using raspberry controller. Software can be causing low performance though.
Raspberry Pi performance may be the problem. The Pi has to check the hash from the cubes.
SHA256 is a very easy hash to calculate. Scrypt is much harder. Raspberry pi takes about 30,000 times longer for a scrypt check, than a SHA256 check.