No, it has to be speed and spin.
This is not only the consensus among tennis authorities, but "heavy" also translates as spin+pace in baseball. A 90mph sinker is much heavier than a 94mph 4-seam fastball because it has a better combination of spin rate and pace.
At the pro level, all shots are very, very heavy. Even Agassi puts 1000-1500RPM on the ball through both wings, and he's definitely hitting those babies flat and hard.