I ride only flat and currently have a 12-27 with a standard crank. I only have this cassette because it came with the bike and I've been to cheap to buy an 11-23. This cassette works fine for me, even though I don't really like the jumps when at the bigger cogs. But anyways, everyone's definition of fast will be different, but when riding at over 30+, I've used the 12 cog a handful of times. I like to spin at over 100 cadence, and my max sprint has only been 38 and was using the 13. Reason I want an 11 is to have more gear options available between the cog, even though I'd rarely ever truly use the 11. But I'd aim for at most a 25. You can play around with this website to get a better idea.