After reading some posts in the Slim Devices forum, I got a Linksys WRT54G router, and the web browser slowdown went away. Also, the SB2 indicates that the signal is 98-100% from upstairs, maybe 25 feet in a line through the floor.
The other router was made by Netgear. The Netgear did 87-90% with the SB2. I tried the Netgear progressively lower levels of encryption, but that didn't change the slowdown. Maybe it was just a bad router...
So, the Linksys works well with the SB2.