This Digibox (Pace BSkyB 2500B) uses an ST20 processor which is built around the RISC concept, so a simple comparison of processor speed with your own computer is not entirely viable; though what we can point out is that the Bush Internet box (with an ARM RISC core) clocks twice as quickly!
On the other hand, the processor does not have to do an awful lot. The slowness is obvious in direct user-interaction stuff, however all of the MPEG decoding and suchlike is performed by custom hardware so the processor speed is not an issue when you are doing what you are doing for 95% of the time - watching telly!