Games Done Quick - Celeste by TASBot, presented by dwangoAC - SGDQ2018

TAS is not "performed" by humans. It is performed by a robot. The game is advanced in frame-by-frame fashion and the optimal inputs for every frame are determined and recorded into the TASbot algorithm. Frame-based nature of the TASbot make duing such runs an extremely long ordeal that can take months to accomplish.
Even then, humans do not "play" the tased game, slowdown or otherwise, they record instructions, relay them to tasbot, check for errors and further actions, advance frame and so on repeat until the run is complete.
The celeste run in question took about a month to record.
The savestates are an emulation thing and could not be used here, since the game is not on an emulateable console.

Let's not make the mistake of thinking all TAS runs are like that, though. Many people have spent hours, days, and months slaving away over a hot emulator, repeating seconds-long segments over and over until the absolute razor-thin time has been reached. Yes, I count myself as one of those maniacs.