Undertale Genocide (Neutral + sans) Route

Ruins grind ends before Blooky, since you skip him if you have 20 kills

Theoretically the best grind is 13 Whimsuns, 2 Froggit + Whimsuns, and 1 triple Moldsmal encounter. This is from going for best average grind, though, so the actual perfect grind may vary

The Ruins, outside of Core, has the most RNG in the entire game by far; that Ruins grind, with 4 frog jumps to end the Froggit turns early, occurs 1/20480000 times. That's excluding perfect step counts. This needs to be bruteforced if you want to get anywhere near optimal

You don't need to sleep before talking to Toriel. Skip the pie and move on to killing Toriel

Snowdin grind is simpler. TAS grind would probably be grinding singles with all Icecaps (plus the first forced Snowdrake), kill Doggo to get to LOVE 8, then kill Dogaressa and Greater Dog in two hits. Though that might be slower than just grinding at death bridge, which is the RTA strat

Doggo is faster if you run right into his attack, kill Dogaressa first to 1-hit Dogamy, manipulate bark attacks from Greater Dog and get hit by the dog to skip his turn. Kill Papyrus

In Waterfall, get the Ballet Shoes, grind after Gerson alternating between the hallway and mush-room. Get a single Woshua after getting the shoes, then after the first 4 encounters in hallway and mush-room, get all Moldbygg and Woshua encounters to decrease the total step count of the grind. Kill Undying

At the lab, there's two options. Get early pan, or quit halfway through, then wait for the camera to lock back on you before exiting the lab. If you just walk out the room immediately you get Alphys's texts, and have to do the puzzles up until the first elevator (https://www.youtube.com/watch?v=dzyaDzY6o38). Which one the TAS would choose depends on whether or not the pan can still kill the Core encounters in an equal amount of turns compared to gun + hat.

From there, just walk through Hotlands, kill Royal Guards, kill Muffet, go to Core. Get the Empty Gun if you don't have the pan

Idk the best encounter to get for Core, probly Froggit + Whimsalot, but they'd have to be timed. Either way, keep grinding up until 33 kills, then go get the scripted encounters on Warrior's Path for the last 7. Kill Mettaton. It doesn't matter if you whiff, that's a rumor. You only abort genocide on Mettaton if you haven't killed all the Hotland monsters, or spared the Royal Guards or Muffet.

From there there's basically no optimization besides surviving sans, and a TAS doesn't need to worry about that.

Neutral is basically the same thing up until Hotland. Get the Spider Donut to skip Muffet, ignore all random encounters in Hotland and Core, obliterate Asgore in 6/7 turns (6 for gun + hat, 7 for pan), kill him, fight Flowey, kill him. Getting the pie for Asgore wastes a lot of time