1.The resource of the web server that your WEBSITE resides on is not enough; these resources might be CPU, RAM, disk speed, Ethernet speed, the network speed etc.
2.The total web traffic on the web server.
Some people say that too many web sites residing on the same web server will slow down your WEBSITE speed; I think that is not always true.
Let me explain: we say there are another 50 web site residing on the same web server as yours, but all the others have very little web traffic, (for example, they are under construction, or has not promoted yet) then they will use very little resource, and they should not affect your web site speed at all.
However, if your WEBSITE resides on a dedicated web server, but has a huge amount of web traffic, then your WEBSITE accessing speed will slow down. You may have heard about mirror WEBSITES, which is trying to separate the web traffic in order to increase the WEBSITE access speed.
3.Poorly developed software, for example, memory leaking or Databases are not used properly.
4.HTML or CSS with too much comments and other unused code. It can cause the final file size to be too big and will take a long time to load.
5.Total file size of the pictures on your web site. All pictures on your WEBSITE should be processed to make it clear enough to view on the screen, but minimum on the file size.
6.Part of your WEBSITE contents are loaded from other WEBSITES and the other WEBSITES are slow.
8.Flash, and video files take a long time to load.
9.The site’s domain name servers is slow.
10. The virus and spy software that may cause your WEBSITE access speed to be slow.
11. Avoid Full Page Tables for Faster Rendering
If you use tables, try avoiding the whole page being one big table. The browser won’t show anything until it’s read the whole thing that way. For a faster loading webpage, either try multiple tables (not nested) or having stuff above the main table to make your content in the first table show up faster. That way your visitors will have something to read while the rest of your page loads. It may not really make you page faster, but it will feel like it to your visitors.
•This is not an exhaustive list, there can be other issues which are not listed above, such as firewall can affect your website speed, but it is necessary; when your web server is down or restarted, then your website will be not accessible or will take long to load. but some times, your web server administrator does need to restart the web server for maintenance purposes.