Put the phone
on the hub that the desktop normally connects to, to eliminate the possibility of a bad antenna or connection in the phone. See if that helps. (If it does, the phone has to go in for service.)
Make sure that the hub's (connect to it using a web browser on the desktop) wifi encryption is set to AES, not TKIP or TKIP+AES. Some ROMs have a problem with TKIP.
Go to speedtest.net (first with one, then with the other) to get the actual speed (it's approximate, and varies from use to use, but "around 20mbps" and "around 5mbps" are different, 20 and 19 aren't). What appears to you to be a big speed difference may or may not be.
As for "what is the problem" - any one of at least a dozen possibilities. Try those 3 tings first. (Create an account here if none of them work - but read
[GUIDE] Troubleshooting Wi-Fi Connection Problems first.)