When using JS isomorphically, the same code can be used to pre-render the page that is sent to the client as would be used to render in the case of a 'simple SPA'. This means that minimal post-processing needs to happen client-side (at least for display purposes), resulting in a faster-appearing page load.
Conversely, in the case of a 'simple SPA' the client-side rendering infrastructure may need to make subsequent requests to the back end in order to retrieve data that is then used for rendering, resulting in a slower-looking page load, as extra details will 'pop-in' after the inital load.
The "Trouble in paradise > Performance" from this article gives a good description of why a client-side only SPA can appear slower, and it's implications.