internet gets slower and slower

My internet browsing gradually gets slower over time. By the end of a day or a couple of days, it is unusable. The problem isn't bandwidth. A speed test shows that throughput is absolutely normal. It's the browser response to navigation. ie clicking a link, typing a URL into address bar, etc. and having to wait minutes for the page to start loading. If I quit the browser and re-open, everything goes back to normal. Any ideas what might be the problem?