Ice boats can see speeds over three times wind speed on a broad reach but downwind they don't exceed the wind speed. Most ice boaters tend to reach downwind, crisscrossing the lake much like tacking but using the aerodynamics of lift to exceed wind speed and arrive at their destination much sooner than the straight downwind route.
I've only iceboated a few times but I can tell ya, scared the hell outta me. The ice isn't smooth at 100 mph and they are so percise one wrong move can have you upsidedown and sliding for a looooong way. I even tried ice skating behind it once ....... ahhhh youth!