No one here can give you the exact "why" of Word processes. We are your fellow users, some with vast experience using Word. We are not its designers.


Track Changes in general adds complexity to documents, a completely different layer of content (possibly multiple layers). When you display it, you are putting extra demands on the display engine. The more of this information you display, the greater the load.

This is made worse by the presence of direct formatting rather than style-based formatting.
Importance of Styles in Word