Connect directly to the modem via ethernet cable and use a program called WinMTR to check for packet loss and ping times.

If that's bad, then you have to do diagnostics with Tech which is a pain.

Ignoring whatever speed tests you have used, if you can't stream 1080p youtube vids on modern hardware, you're having packet loss issues somewhere.

I have a 25mbps service that I share with 7 people and everyone calls it 'fast' without realizing that with my G network, they're really only getting 14mbps max. Low latency and zero packet loss are the most important things for general surfing and even streaming. It's only the bulk downloads that really 'clog' the system.

You've also got to factor in that practically speaking, Wifi is only for convenience and wired is for speed and reliability (unless you're truly comitted into buying commercial grade wifi antennas).