The music wasn't laggy. It flinched every time you landed.

The report back after the new soundtrack shipped: the music "seems to play in a laggy way during game play." Two suspects — the CPU cost of the new engine, or the system that ducks the music under sound effects — and only one of them was it.

Not the CPU

Frame times measured with the music on and with it off, over thousands of frames: identical, mean 6.06 milliseconds either way, no long tasks. The audio engine itself costs about 5% of one desktop core, which was already known and already small. That ruled out the obvious explanation in an afternoon.

A tremolo locked to your own landings

Every sound effect was calling the same function to duck the music out of its way — land, perfect, and its echoing octave all fire on one landing, each dipping the music 42 to 50% with a fast attack and a half-second recovery. Landings happen every 0.3 to 0.5 seconds during a real run. That's not an occasional dip. That's the music being pulled down and let back up on almost every single beat of actual play — a rhythm nobody wrote, locked to the player's own landings instead of the song.

The fix narrows which sounds get to duck anything at all: only one 0.3 seconds or longer — a death, the RUSH whoosh, an escalation, the last note of a fanfare — dips the music now, by at most 30%, no more than once every 0.6 seconds. A landing, a coin, a click, a bank, a dash, a chirp: none of them touch it anymore. What felt like lag was never the engine falling behind. It was the music being interrupted by the exact thing a player does most.