Comment by drKarl

With server side rendering you have to reload the whole page everytime there's an update on some data or if you want to submit data. That is not only slower but it also feels slower to the user. With SPAs you only reload the part of the page that has some data changing which loads asynchronously in the background, so it's not only faster, it also feels faster. If your new implementation is worse maybe the company you hired didn't do a great job...