And if you broaden your idea of your user base a bit, even the fast computers aren’t ubiquitous. Try using a low end Android phone sometime, or a hand me down iOS device still on iOS 12.
a site where every interaction requires an http request and a reload of the page performs much worse over low latence than an SPA that is designed with a local first approach where all interaction is local and data requests happen in the background.
not all SPAs are designed like that. but none of the traditional sites are designed like that either because they can't.
take hackernews as an example. in order to write a reply i have to click a link, wait for the page to load, click submit, wait for the submission to complete and for the updated page to load.
i live with bad internet. submissions and pageloads on hackernews frequently time out.
with an SPA the first click would open a text field without any server request at all, and submit would happen in the background while i can keep reading. and it can be repeated if it fails without me noticing and having to worry about it.
likewise updating for new messages could happen in the background without me having to wait for a page load. on top of that new messages could show up in a different color without having to track that in the server.