This could be caused by a number of different things. My recommendation is that you start here to identify the point at which it becomes slow:
The result of your test will determine what to do. If the slow point is before anything starts to load, you may need to optimize your web server or web application. More often than anything else, the answer is to optimize your web application by reducing the amount of effort required by the server to generate a page to load.
An example of optimizing a web application is to use Wordpress, since it is most common. In such a case, reducing plugins and using a lightweight theme, as well as using a static caching plugin, can help with that.
If it isn't at the beginning there, you should be able to see that it is specifically something else. If it's an external call, it may be another server running slowly and you may need to pull the content locally. If it's something like a large uncompressed image, then you can compress the image and re-upload it.