Why does my iPhone run slow?

I have an iphone 3G currently on 3.1.3. I downgraded from ios4 because it was running incredibly slow, so I decided to downgrade to 3.1.3. But it is still slow with huge lag times, when opening an app.
What shall I do?
I have tried hard reset as well to no avail…

Depends how long you had it. Try deleting all you're history go to settings> safari>advanced> delete all history> and their hopefully it works a bit

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