Many factors are involved, but when simplified it's just the maximum damage you can deal at the earliest part of the animation.
Wpf/weapon speed lets you reach it early. Movement helps you get to the optimal hiltslash location easily with your quickest swing and adds damage with speed bonus along with PS. As you're probably well aware, it's a bit of everything. If your weapon is extremely fast, you don't even need to worry about hitting as early as possible and decreasing your damage from sweetspots.
For melee damage, the damage in the calc is on the low end. Speed bonus raises my 57 raw to mid 70s on an unmoving opponent. It could possibly double if we're both moving towards each other.
~130% raw for a good speed bonus hit
Up to *125% raw from a held swing
110% from an overhead
1.2x final damage when hitting the head
If you go from 60 base raw base to 80 on the calc, with similar modifiers it offers a very high damage potential to the strength build if he can use his opponent's speed against them and make use of the momentary speed increase when turning. This is all still fine imo since it's much more difficult to 1shot someone over 3-5 shotting. As you know, it's also much harder to get all of those bonuses properly but packs a punch when it does, unlike an agi build that relies on them for decent-good damage.
