Hey!
As with anything involving wireless...
1) Check the signal strength. If you have one out of three or four bars, this can cause the slow internet and frequent dropping.
2) Double check you have installed the drivers for it. I assume the dongle came with a driver disc. Make sure you have installed any required drivers from it. Otherwise, search for it on Asus's website.
3) Are there any other devices that could be causing poor performance?
Having lots of other nearby routers, or lots of devices connected to the same router can cause what you are experiencing.