Above 1500 m, your VO₂ max drops by ~2% per 300 m. A race in Mexico City or Boulder runs differently than at sea level.
Arrival timing
Either race within 24 hours of arrival (before acute mountain sickness sets in) or stay 10+ days. The 2–7 day window is the worst.
Pace adjustment
Expect to add 3–5% to flat-course pace at 2000 m, more above. Heart rate runs higher at the same effort — don't fight it.