I question the wisdom of the LBS suggestion. A thorn and an nail are pretty different unless it was a really small nail. Are these tubes self-sealing like with Slime? That might help for a while - maybe enough to get you home.

I'm more of a fan of puncture resistant tires. If the nail or thorn can't get to the tube, the tube doesn't have anything to do except its normal job of holding in air.

As far as the 1 mph difference, maybe you were just having a bad day.