Is it slow because there's a lot of data in total being transferred, or does CSMA just collapse (gridlock) so hardly anybody is getting anything?
Usually it is because of data. But not necessarily that simple.
You can see 16 SSID's but if no one is transmitting traffic right then then there shouldn't be a problem.
If someone else is transmitting data then it depends whether he is on the same channel as you (or one that overlaps yours). With the 2.4GHz range there are only 3 channels that don't conflict/overlap each other. 1, 6 and 11. If he's not on your channel then there shouldn't be a problem.
Finally, if he is transmitting and he is on your channel then the issue is whether he's using enough broadcast power to cause problems with your communications. Basically, a WiFi channel is the same as a bunch of people sitting in a room and yelling whenever they want something. The more broadcast power, the louder he's yelling. And most people crank their systems up to max power if that's an option.
There is also another possibility. Anything that uses electricity can generate radio frequency "noise". Such as a microwave oven. Which can also cause problems with WiFi transmission.