0.5g of ferric nitrate speeds things up noticeably; two more spoonfuls makes it even faster. Cerium ions don’t show much color, so I just rinsed briefly with sodium bisulfite. If you use a lot of iron(III) ions, EDTA may be needed to prevent iron(II) ionsresidue and subsequent discoloration.
If the speed is too slow after using, it might be due to not enough sulfuric acid—some free acid is needed to solve silver. Glafkides’ original formula may not have enough sulfuric acid, which limits capacity (and he got the valence wrong in his book). A little cysteine might also help, but I haven’t compared yet.