It seems to me that one reason the battles seem faster is because the Japanese armies move in a more orderly fashion.

In VI, light cavalry and horse archers are quite fast, whereas knights and heavier cavalry and heavy cav archers move closer to the speed of STW cav (excepting Yari Cav, which are quite fast; more like VI light cav).

I think the main thing that slows down VI armies are the Pavs--they are very slow, both moving and forming up. They don't react nearly as fast as musketeers and STW archers when ordered to run or retreat from charging cav or infantry either (those big shields are heavy and unwieldy).

Maps in STW are smaller, so it would seem that this would also contribute to the speed of battles.

One difference that contributes to VI seeming faster in one way is: cav in STW seem "stickier." Hit-and-run manuevers must be timed much more carefully in STW to avoid a prolonged melee after impact.

One thing I like about STW that doesn't really have much to do with the speed of battles--Yari Samurai are very effective spearmen, whereas VI spearmen variants tend to get beaten a lot faster, even by some of the cav units.