Finished the game yesterday. The hardest time trials for me were:
- Playground 3
- Atrium 1
- Atrium 2
- Shard 1
- Actino Rise
These were also the ones i chose to 2-star for the Superstar achievement.
The only speedrun i had a hard time with was 9, The Shard. Overall i found the speedruns to be pretty doable, definitely easier than the time trials. Sure, they are a lot longer, but they are also a lot less strict on time. For the most part you don't have to pull of any super fancy moves, you can just take a route you are comfortable with.