Does she ride a lot or is she just a casual rider? I'm faster than all my friends and it seems like I have to go agonizingly slow when I ride with them, even though it's just 2 or 3 mph slower than I'd be going normally by myself. So her being slower than you doesn't mean much.

Nor does you "feeling slower" on her bike. The perpective is different. The farther from the ground you are, the less the sensation of speed.

Unless you have a cyclocomputer to compare speeds, or have tried out some routes you are familiar with your usual times on with the new bike, I'd be inclined to say it's nothing but your perception of things.