Use PageSpeed Insights to test your site speed as loading speed is a factor in search engine ranking. PageSpeed insights provides specific recommendations to reduce site loading speed for mobile and desktop.
Image and file sizes
A common mistake is to aim for the highest quality in images. In general computer monitors and devices display at 72 dots per inch (dpi) so there is no need to save your files at a higher resolution. Reducing the dimensions for your image also reduces the file size, so only make it as wide as you will need on your website. JPG files offer a lot of control over the compression which can further reduce your file size and normally you can set your quality at around 65 to get a fast loading image that looks good. Using PNG files is also popular because they offer the option for transparency, but file sizes a re generally bigger with PNG than with JPG.
If you are posting PDF files, optimize them as well for fast loading. Adobe Acrobat has features to reduce your file size that make a big difference. PDF files do come up in search results so be sure to also optimize your pdf files by adding a unique optimized title in the document properties.