SPAs are faster than SSR because you can choose whether to incur network latency or not.
For example, let’s consider a site with two pages. In the SSR case, if I click the link to page 2, then I have to wait a whole network roundtrip for the page to appear.
In the SPA case you can decide when you want to load the content for page 2: with page 1 (if it’s a small amount of content), if the user scrolls to a certain point, … it’s completely up to you. You can cleverly hide the latency so that when the user does navigate to page 2, it’s instant.