i don't think so this matters much ,I suggested atcoder contests to work on speed and it doesn't matter where you practice what matters is that you solve questions which are good. In my opinion codeforces is a good platform to practice.

first of all don't go behind too hard problems or too simple problems try to find out the rating level question which you are able to solve during contest duration. solve question with rating up to 200+ the the rating you are able to solve during contest.

for speed i will suggest you to go for Atcoder beginners contest they are comparatively easier and will help you build your speed. you can also try giving virtual div3 on Codeforces. hope this will help you.