This PS/2 keyboard accepts a maximum of 2 arrow key presses at once – if you press more, the PC speaker beeps and no keypresses are detected. The beep is sometimes 1 second in delay.
From 1:52 to 1:57, I switch to my normal (Logitech G15) keyboard to see if the simfile is actually in-sync.
The song is dBu’s remix of 「月時計 ～ ルナ・ダイアル」 (Lunar Clock ~ Luna Dial), the theme of Sakuya Izayoi, the stage-5 boss of Touhou 06: Embodiment of Scarlet Devil. The StepMania theme is CyberiaStyle 4 for StepMania 3.9.