|"I had some packet analyser program but not sure how to use it or read the info"|
Another packet analyser isn't going to help in this regard.
"when I tested the download speed but very fast if I plug a computer directly into the router"
You don't have a bandwidth/download problem. You have a wireless issue. You would have a bandwidth problem if it continued to be slow with the direct connect.
Now if this isn't a home but business setup this can be a different story.