I lost count on how many Android devices came out after the 3Gs, why would you be surprised that the Droid & Nexus have better hardware specs?
I have yet to have one slowdown on my 3Gs and I don't see many topics like that popping up in the iPhone forums. Yet, I see a lot of people suggesting "App Killers". Supposedly, Android devices do not need it, but if that was the case, it wouldn't be being suggested at all. It's obvious that some users are experiencing these slow downs, including me! I've had to restart my Nexus One more than my BlackBerry 9700, which is surprising to me.
Every phone has it's ups and downs... no phone is perfect or I wouldn't have four different OSes in front of me right now.