A very common issue that clients have is a slow loading website. Most of the times when its not a bad combination of plugins causing a slowdown, or a theme that is just poorly programmed, then its caused by HUGE un-optimized images. I cant tell you how many times I have looked over a client site and seen 2-3mb images being used over and over again causing 6mb+ page sizes in images alone.
When images are the cause of the slowness that clients are having with their site I always make sure to recommend that they first and foremost make sure the images that are being used are not scaled via HTML, but rather they are indeed uploading images of the correct size. This in itself can speed up the loading of the image, as well as reducing their monthly bandwidth charges (In some cases saving them a lot of money in hosting, and/or CDN costs).
Another thing I like to do with all of my images before I upload them to my site is running them through JpegMini as well as ImageOptim. Using both of these programs hand in hand can greatly reduce the size of images on their site.
Lastly if they do not have the money to pay for a CDN service I always recommend that they install the JetPack plugin, and then enable the free Photon CDN module. Note: the only caveat with enabling this module is at times I have seen it cause some slider plugins, and themes with built in sliders to stop working.
Overall when customers make use of these steps they should see a faster load time on their site, and generally a reduction in bandwidth usage and charges because of the optimized images.