I have a samsung galaxy s3, and when I'm connected to the WiFi its slow. I mean my friends would come and use their iphone 4S and it won't lag on there phones.. so is anything wrong?
My guess is that unless this is something that has been going on from the beginning you might want to look at your router as being a possible problem. Different devices react differently on the same router so you may want to start out by powering down the router for a minute then powering back up again. After its up and running hold the power button on your S3 long enough to be able to tell it to restart. If that doesn't seem to help, find your connection under settings-WiFi and Your connection, tell it to forget, then reconnect.
There are other things you can try if you feel like it has been slow ever since you got your S3, but try those suggestions 1st.