I live at 150m above sea level.

I've ridden my mountain bike close to 2500m, and for more than 2 hours without descending under 2000 with no issues. I dind't feel any difference, and according to my heart rate monitor I had my usual heart rate. One of my mates said he noticed the altitude, but he rode at his normal pace without issues too.

I suppose it's something that changes from one person so another, but 5000ft are only 1600m, so I doubt you'll notice anything at all. It's a farily low altitude after all.