My splits explained: I split at world 1, world 5, 8t, 8c, 8t2, BC/END. I split when the black screen after completing a level fades out.
My comments and times (as I record them during my runs) on each level:
1-1: Did RNG manipulation. Last two digits on timer were 70. I recorded them during the run so I'll post them along with my comments on each level as well. 1-2: Didn't get blue shell from the red block. I've gotten 60s before so not great. 58 1T: Screwed up the wall jumps a bit. 46
1Cannon -4:06- Slid off the cannon. Bad. 96
5-1: Somehow got a pixel stop. 63 5-2: Got another bad pixel stop. 60 5-4: Unfortunately got the hammer bro. 64 5GH: Mistakes with vine and bruisers. 15
5Cannon -8:47- Didn't get any TAS cannons this run, though I usually get them easily. Disappointed in that regard. 97
8-1: These bloody pixel stops this run. I have no idea. 68 8-2: Didn't get the barrel jump, and I haven't even practised that new shell jump skip yet so I didn't do that. 42
8T -12:19- Safe kill gone bad. Luckily didn't lose the shell. Got the mushroom house for double digits though. 22
8-3: Not bad, some optimisation to be done. 300 8-4: Screwed up taking the hit on the spider because I do fire flower strats. smh -59
8C -16:32- Not used to having the shell here so I quickly fell in the lava. Second go I missed -67 so that's dumb, but luckily didn't have to scroll back because I already got fireworks in 8T
8-5: Died once. Then -51 because I'm bad. 8-6: Hard level to do optimal. -54 8-7: Admittedly I don't have a strat for the level. -47 8-8: The star block broke from a meteor just as I hit it so I couldn't collect the star! Luckily I didn't take too much time making it to the end of the level without it. -69
8-T2 -23:38- Did the death skip, 382 so not quite optimal fire kill on Jr
BC/END -26:57- Died at bowser jump once but got it the second time.

Earlier today, Twitch has that they are "implementing a 100 hour storage limit for Highlights & Uploads starting 4/19." There is a 2 month grace period for people with highlights on their channel to be able to download their highlig