You're not the first person to notice this. Slow download results are caused by heavy traffic on the server. Or a gazillion other reasons. Since the test is free there's not much you can do about it. I use this site and pick a server close to me (Dallas) and get the results my cable company advertises.
You need Flash to make it work.
I just upgraded my DSL connection from 1.5 Mbps to 7.0 Mpbs and wanted to test the actual bandwidth. I've run tests at several web sites including CNET. CNET always returns results of just 1.3 to 1.8 Mbps while all the others test sites return results of 4.8 to 5.2 Mbps.
All tests have been run using a desktop PC connected to my router/modem via ethernet cable. Any thoughts as to why CNETs results would consistently vary so much from the other sites?