Static meshes, depending on mobility, will render much faster.
Overview of mobility:
Actor Mobility | Unreal Engine Documentation
General performance guidelines:
- only use skeletal if you need keyframe animations
- make all static meshes the lowest mobility required
General attachment guidelines:
- you can attach static to skeletal, and vice versa, but …
- static meshes only attach to static mesh of equal or lower mobility
- only attach “movable” static meshes to skeletal meshes