Site speed ups can be very complicated and involve a lot of tweaking, but if your site is loading really slowly have a look at the filesizes of the images on your pages.
A large filesize will slow your site down considerably, particularly on mobiles which have restricted bandwidth. You should be aiming for images of less than 250kb, preferably less than 100kb.
The best way to reduce the filesize is to use a photo editor on your desktop to reduce the actual size of the image, preferably to the actual size you need it on the site – so typically no more than 800px high.
You could always try install an image optimizing plugin such as Smush or EWWW. These will go through the images you have as well as any new ones and strip out any unnecessary data from the image.