I recently did some optimisation work for a charity website here in Dronfield, they were complaining that there website was really slow.
There are a number of common reasons for a slow website, this can be anything from slow hosting, no cache, a poor theme or lack of optimisation on the back end.
I found that there biggest problem was images on the backend, these days you can use flickr or other places to store big photos. Big bulky photos will slow you down especially if you arent running a serious image compression plugin.
I compressed over 500 images but was frustrated that in media library there is no way to order in file size so I set about finding the best plugin to let me achieve this.
I tried 3 or 4 different ones but the best I came across was one called Media File Tools this is a very lightweight plugin that adds the option for size and to sort by MIME type.
With this option ticked you can keep a very close eye on your library and see if anythings uploaded which is too big, very handy if you have a few different admins.