The demands of web browsing on your computer’s memory and cpu should normally be extremely low (and you can monitor it using the Task Manager, hit Windows key + R, type taskmgr, check the Performance tab). The "slowness" or "freezing" you experience from time to time usually has to do w/ factors outside the confines of your computer, such as an overloaded website server, heavy traffic on the internet generally, or the fact your web browser has to retrieve many separate elements (text, images, flash, javascript, data from a database, etc.) from the website to build the page, perhaps even from other sources *it* doesn't control (e.g., ads!).
Now that’s not to say you couldn’t be having problems locally, esp. if it’s ALWAYS behaving this way, and not just on occasion.