The ? in the URL is used to ensure a unique URL when the file changes. In other words, when a new version of Q2A comes out, your users will get the updated Javascript.
It doesn't cause any slow down itself, if your files are loading slowly it's likely due to a slow server.
Edit: however I did notice on your site you have the qa-styles.css appearing 5 separate times in the HTML! This will cause 5 different requests, so you should fix your theme to ensure you're only loading the CSS once.