I don't like to look at from a MA style point of view. I like to look at from principal point of view. 6 Harmonies principle is above all MA styles (include XingYi).True, but since the question was about Xingyiquan……
- Taiji trains in slow speed because it's easier to coordinate your body parts in slow speed.
- Baji use heavy foot stomping to train body coordination.
All MA system try to achieve body unification (include XingYi, Taiji, Baji, ...). They may just use different training method.