Capcom's in-house engine, MT Framework and it's successor, the RE Engine. Both for their ability to render well and fast, and also handle the movement of entities who are currently un-rendered or unseen. This is significant because a lot of engines made in the early to mid 2000's tend to idle entities that aren't directly seen as a way of saving resources, and this means that in some action games in the past, keeping an enemy off camera so they will idle was a valid tactic for managing combat. It's not something you see these days, as computers and programs have got faster and more powerful.
Unreal has dominated the action game space for such a long time, it was a big risk for Capcom to develop their own.
