What UX research suggests that most users encounter “unresponsive” webpages thanks to the backend roundtrip issue? AFAIK, this roundtrip issue typically adds milliseconds to the responsiveness of most conventional web applications. Most users will not be negatively impacted; most users won’t care, as Drew suggests.
I’m not saying that SPA doesn’t have a place in web development. I’m saying that most web applications simply do not need to be SPA. Show me evidence to the contrary.