Why is my wifi so slow. My linksys router one day just quit working at full speed.?

This is wierd because I ran a speed test app on my iPhone and it was normal, also I have full bars. It isn't our ISP because the hardlined computer is working normally. It's just that the wireless is SLOW. For instance, on YouTube, it takes five minutes at least to buffer enough of a 1 minute video to watch without stop. Can anyone help. I'm so sick of it.