Performance speedups are dependent mostly on possible parallelism and implementation. Schiwietz et al report in "mr image reconstruction using the gpu" an 128x improvement in using a GPU instead of a CPU for CT reconstruction. However; Kachelreiss et al's has a version of the same reconstruction algorithm which runs twice as fast on an intel CPU as Schiwietz et al's gpu version; even though he was optimising for the CBE (Hyperfast parallel-beam and cone-beam backprojection using the cell general purpose hardware)