Rowlet is more of a speedy pokemon than Popplio is. Popplio looks like the kind of pokemon to get slower upon evolution due to gaining size. Rowlet probably gets more speed when it's wings get bigger and is able to use them more often. Litten might be fast as it uses all four legs to run and maybe it stays on all four legs throughout evolution. Legs might get longer so Litten can run even faster.
I think it'd be really cool (and kinda funny) if Litten ended up being the fastest and going a king cheetah route. They already have stripes, they'd just have to make them thicker and blotchier.
Popplio seems kinda like a Special tank, tbh, and I can see Rowlet going more for a glass cannon type of deal (unfortunately. Darn you, Ice types!).