if the above is true, why does my phone get slow when lots of apps open (by themselves) and use up memory, then go fast again when I kill those apps?
Dunno. Maybe you have an errant app that’s misbehaving. If you’re running Froyo, this seems plausible; a third party task killer won’t really ‘kill’ an app under Froyo IIRC, it’ll restart it.
So when you say “those” apps, perhaps it’s one in particular that’s causing problems? Something like Watchdog may help you pin down what’s going on.
You don’t have “lots of apps open”, it just doesn’t work like that. Generally, background apps are suspended and will be unloaded by the OS if the memory is required for a foreground task; the exception here is things that are specifically designed to run as a background service, say a GPS tracker or an mp3 player.