I have read of several people who put these flat-resistant tires on their bikes and then complained about how much it slowed them down. Check your tire brand and type.

A friend of mine and his wife both had mountain bikes and were similar sized. I don't know that they had a difference in riding speeds, but one of the could coast noticeably faster or farther than the other when riding together, and they never did figure out what that was.

You can get the bearings too tight on wheels and bottom bearings (at least on some of them). That can cause some extra friction, but usually has a sort of gravelly feel in the bearing as well.