You are connected, but the site that you are trying to get to does not exist, or is down right now. Try accessing another page. Check or retype the address. The site could have been shut down or abandoned. You might want to contact the webmaster of that site.
Some sites can be slower than other for a few reasons.
If they are really far away, then it takes a bit longer for the information to be transferred. Also, it depends on how fast their link to the Internet is and how many people are 'hitting' their site at the same time that you are. So really, speed of web pages vary, and the reasons vary. A lot of the web sites that offer software downloads have mirror sites for you to download from. What a mirror site is, is a copy of the web site at a different location on the Net.