Cosmic Plains is the first planet in the game so you can tell if a run is good from the start so the beginning will probably in the future be optimized the most. The first question in the run is "is going on the right side of cosmic plains faster or slower then the left side?" Now if you look at the world record by 4Mahoney21 they go on the right side and his cosmic plains takes 2:20 but the second best run by Paul1379 done by going on left side first and they take 2:17 to finish. In conclusion, for now left side first is faster.

edit: their new runs have them swapped but still faster

edit again: this is outdated and a new guide by P4radox has been made so look at that instead.