1. Refer to this blog and learn how to reduce the image sizes first:
Do you love taking pics with your mobile? Great! Just DON'T upload them to your website without referring to the blog above to reduce the size of the image.
Images are the biggest cause of poor load speed of a website. Each time a large image is added to a page or blog on your website the browser has to render the image to load it for each new visitor, and on Mobile, that takes even longer to do. So if the image is large e.g. 1MB+ you'll have issues.
Many websites suffer these issues and although Google's preferred speed levels have not changed (80/100), their insights scanning tools have definitely become smarter in picking up these stats and flagging them as an issue.
Do Plugins Work?
In our experience, WPSmush and similar plugins may help a little but still don't offer the best results. A Speed Upgrade service needs to be scoped, quoted and undertaken by an experienced code/web developer to ensure each issue is managed - server issues, images, Java, CSS etc. - based on the results from the Google Page Speed Tool. A standard WP web developer may not be able to assist with some aspects in full so typically we use a coder/web developer to manage the lot.
However, that's doesn't guarantee a permanent fix. It really comes down to the WebMaster understanding the need to optimise image sizes BEFORE adding them on the website. If large images are continually added you will have the same issue within weeks/months.
Google Page Speed Tool