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 is just barely powerful enough to run a full bank of Bitfury cards. And bitfury's design is as simple as it gets. IIRC connecting to an active miner for monitoring was enough to cause minor performance issues. Raspberry Pi is also known for corrupting it's SD card very quickly. So in every way it's a very poor choice for a controller.