I noticed the same thing at the AT&T store today. I had the Surround and the Focus in each hand on the exact same web page, and the Surround was about twice as fast to change orientation. I tested this on multiple websites, multiple times. How is this possible - I thought all WP7 phones had the same processor/chipset?
EDIT: I just thought of something. Maybe the Focus isn't lagging due to processing speed, but rather because the accelerometer/gyroscope/whatever sensor is wacky?