Iphone 3G getting slower

I have an iphone 3g and it is still in perfect condition. I don't understand why it keeps getting slower and slower. Every time I open an app that requires the internet, it has to be opened twice for it to work. I swear that ever since the new 3gs came out my phone became a lot slower. I've tried restarting my phone multiple times but that does nothing.

Anybody know what's going on?