I just stumbled on THIS interesting post in a forum.
Mainly the middle part, about Hosts, caught my eye. The "hosts" file is what Spybot SD uses for its "immunize" function, and it currently has almost 9000 sites in there for blocking (mainly porn, I guess).
Anyway, I haven't got what I'd call a "slow" computer, and 9000 is still a way behind the 16000 number that 'Karl' mentions - but I figure if there's a chance of my 9000 slowing down my browsing by ANY appreciable amount, I'd rather not.
So, I've run my SpyBot again with Immunize "UNDO", so my hosts file is again empty. I can live with the (tiny!?) risk of having my browser go somewhere I might not want! I'll handle THAT when and if it happens. 
