Domain-wall mobility measurements have been carried out on picture frames made from single crystals of the composition Y3Fe5-2x3+(SiFe2+)xO12 in the temperature range from 77° to 300°K. The geometry of the picture frames was such that a wall parallel to the 110-plane was favored. For x = 0.04 and 220°≪ T ≪300°K the mobility R can be described by R = R0 exp(-E/kT), with E ≈ 0.4 eV and R0 ≈ 6×108 cm sec-1Oe-1. The lowest mobility observed is 1 cm sec-1Oe-1. The values of R0 and E are consistent with loss measurements and can be understood as resulting from a diffusion aftereffect involving electron transitions between Fe2+ and Fe3+ ions. Below 220°K this heavily impeded wall motion could not be traced further, and a faster mode with R ≈ 103 was observed. The fast mode could also be observed above 220°K in applied fields surpassing a threshold field Ht. We identify this fast mode with the one observed by Wanas. The transition between the two modes can be described by Janak's theory of diffusion-damped wall motion.