But a lot of the time they can happen in the background where the user doesn't see them, or on a part of the page the user isn't currently interacting with, preventing it from being a stop-the-world interruption.

It’s in the name: an MPA requires a page roundtrip for every navigation, while SPA requires a single page roundtrip. Any subsequent requests are part of the application itself and can be handled without disrupting UX.