Why website is slow?

There are some reasons:

1. Unoptimized Images
   Full size images or images are not cropped may causes the slow page. We should use optimized images in the web pages.
2. Dynamic Script Page
   Pages that are generated from the data stored in the database are slower than Static pages. If data is taking time to get the from the database we should use caching.
3. Bulky Code
   Bulky code can be white space in the page, inline style, external css, external javascript,  Div based layout instead of table based layout, using css shortcuts.
   We should avoid these as much possible.
4. External Media
   Slideshow, flash, movies.
   if possible, we should store them own server.