I want to say range is closer to 25-50 meters, or basic wifi strength. Streetpass can take anywhere from .5 of a second to 1.5 (may be higher, I didn't sit with a stopwatch) depending on how many games use it.
Streetpass can go through walls, depending on how thick (again, just like Wifi). The more wiring/metal in the wall, the less likely to get through.
A metal lunchbox will shield a 3DS from getting any hits. Discovered that one day when bored.
Generally, if you get a hit from one 3DS, that unit won't give you another for at least 6 hours.