If you're offering a service that is so uncompelling or so undifferentiated that the page load speed is a driving factor, than you should certainly focus on page load speed.

On the other hand, Gmail regularly takes 10s+ to load and yet most users are glued to it.

If you're a SaaS business, your marketing page is probably not your app. The people using your app care about usability, and page load speed is rarely the driving factor. Especially with SPAs.