I've been living in the lowlands for the past 7 years and this summer went to CO for a couple of weeks riding 500+ miles in the mountains with many days well above 7,500' and climbing passes to nearly 12,000' (Loveland pass 11,990 :-). At the start of a ride I started breathing heavy sooner, and it slowed me down a mile or two per hour on avg, but other than that I didn't notice any effect. It really surprised me as I was expecting the altitude to have a greater effect.