My custom cassette had one big jump that was starting to annoy me. From 26T to 20T made it really hard to get up to speed. In everyday rides and brevets it wears me out too much. I worked on the cassette (had to take out the wheel completely) put in a 23T and took the 19T out. So now my cassette is 13-15-16-17-18-20-23-26-30-34 with 70/39 in the front and 130mm cranks. What a difference that makes. The acceleration between 34km/h to 42km/h used to be sometimes a bit of an agony but now it feels natural. I'd like to also have the 13T changed into a 14T but that will not fit. It's not all that crucial since on the 15T I already ride 55km/h and a 14T or 13T is rarely needed. The 13T is now sometimes used as an "overdrive" downhill.