For my experience with the Steam store at least, I think it's a difference in expectations. The Steam store isn't a single-page app, it requires constant page reloads and navigates like a traditional web site. That kind of click-wait-click-wait is jarring when you're moving between more fluid navigation in the native applications running on your desktop (or the Steam wrapper around the store to launch games, etc.).
To me that's why it's clunky in the application, but fine in a browser. It's par for the course in the HTML/native hybrid application world.
There is definitely a _huge_ difference in performance for the same Steam Store pages between the app and looking at the same page in a browser. Seems like something is poorly optimized in the application.
For some reason I can't reply to winslow, so this goes here: Yes, you can open tabs. But that's rather useless (unless you want to use it as a general browser). I phrased that badly. What I meant was that you can't open links in new tabs. Which is what I'd like to do when browsing through sales or genre lists.
If you are talking about the in game Steam browser (based on chrome) yes you can open new tabs with Ctrl+T. Though their desktop version isn't meant to be a browser hence the limit to steam store and on tab.
Fine in a real browser though.