If it is really lagging then try a restore, seems to fix a lot of the lag. It will delete all media though, just a warning. And back-up before doing it so your data will be put back on
Even if it was a refurb they sometimes are better then the new because apple tests them to me sure that they are working. If it came out of a little white box it's a refurb. You should google 3g/3gs boxes you'll see what the new ones should look like.
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