We've been experiencing sluggish pages since long before JS really became a thing too. PHP and a slow inline query to render results has been a possible problem for literally decades.

Do you investigate all the fast snappy sites you visit to confirm they aren't single page apps? What about the slow pages that aren't SPAs (most every news site)?

It seems like anyone who wants to dislike JS and/or SPAs can easily confirmation-bias their way into believing it.