I'd hesitate to call this "optimized", but this is the only thing in this game that I came up with by myself, and it seems to be faster than what other people are doing. I have used this in all of my PBs since then.

You want to make mental notes of which planets you're going for, and adjust your turning wide or sharp to get there. If you're slow then the racecar planet will run into you at 20sec (free size), but you can tell you're on good pace if you beat it there, and you don't need to collect it to finish. If you want "marathon safety" or whatever then you can get that smaller one right after the Big Bang, but once you understand what you're collecting in this route, you'll know that you don't need it (just don't miss anything; I have another clip where I missed 1 planet on this route and failed the rollup). I went through several vods when getting this clip, and this route consistently finishes around the same time with only ~3 seconds of variation.

What we know about this level: -It behaves like a standard size level, despite the fact that both your current size and the goal size are hidden -Planets have different sizes just like normal items -Being the bare minimum size to roll up the Big Bang is not enough to collect the Sun. However, being slightly larger than that is enough. -You can't judge your size on object count alone due to the stardust, which adds an object but virtually no size. -It's not likely to happen on the speedrun route, but it is possible to collect objects while you're on the rainbow going towards the Sun

If you want to practice this level or improve the route, what I did was play the speedrun up til the end, then make a save file when you're at this point. Just reset during the credits after you get the sun. You can't do this on a completed file because there will be additional stardust throwing off your size. I'm sure that this can be improved a ton because there's still a LOT of dead time in here. I stopped trying to route it as soon as I memorized this one because I just wanted SOMETHING consistent, but I didn't spend additional time working on it after that, so this was just like a first draft.