Optimize Images
Images often make up the bulk of a page. That’s natural since they need more space than text or CSS. For that reason, in order to speed up WordPress, it is crucial that you learn how to make them as small as possible without losing quality, or what is called compression.
The old way used to be compressing your images before you even upload them. For example, in Photoshop you can use the Save for web option but this will not compress the images to the maximum. TinyPNG used to be one of the best image optimizers but you will probably get much better results with ShortPixel and its lossy image compression. It will automatically compress your images when you add them to your WordPress site. It also has a bulk optimization tool so you do not have to optimize your existing images manually.
If you prefer a free plugin then try WP Smush Image Optimization. It will compress any image you upload to your site but the results will not be as compressed compared to premium image optimization services.
You can also use a free online image compression website like compresspng.com.