It focuses more on its beautiful resistences instead of speed. Resisting Quake/Edge is extremely good. Virizion is often too slow to outrun the things which can beat it (Latios, Latias, Scizor etc), or other things which beat it that are slower (CM Wish Jirachi, SpD Jirachi, Reuniclus, etc.) A lot of the pokemon Virizion can outrun with max speed it doesn't need to and you can cover them otherwise.
Impish Breloom worked, didn't it? Virizion has even better stats and can boost. Good asset. It can even beat Chansey. It also beats Bronzong with much more ease since Gyro Ball doesn't do enough.