Can you still watch Full HD videos (especially the 1080p60 videos) without any problems? If so, the problem is not with you, but with the website you're visiting.
You've got to know that Steam is a huge company and has the resources to upload at super high speeds, so is YouTube, which is why I asked if the videos there play fine as well. I always draw max speed from Steam when downloading games (200mbit or about 25MB/s) but downloading from websites or visiting some websites takes ages. That's because their upload speed is simply slower.
That being said, try doing a speedtest. Usually your internet service provider has one, otherwise speedtest.net can help out. If that shows speeds that are close to what you pay your internet service provider for, then the problem is unlikely to be on your side.