That's an excellent question! In general, if your site is running slowly, we can always advise you on whether adding more worker processes will help. We've got some work in progress that will automate that at some point in the future, at which point we'll be able to send you an email suggesting an upgrade when will be useful.
For right now -- I took a look at the load on your site (I'm guessing you're talking about the custom domain one) and it doesn't look like upgrading will get you the most bang for the buck. Instead, try using the "static files" table to specify where your static assets -- images, CSS and JavaScript are. That should speed things up substantially.