Help Diagnosing Slow WiFi issues

We have a 200 down package, and when I did a speed test over Wi-Fi, I was only getting 3 down. Via ethernet the speedtest gets 70ish down. I know it'll be a little bit slower over Wi-Fi, but not that slow. I was hoping for suggestions as to how to find out the problem. Using a Motorolla SBG6580 router/modem. Thanks!