Second question: Is it important what the site webserver is to choose a cache plugin between Litespeed Cache and Rocket?
Because I heard that the Rocket plugin is not suitable for sites that have a Lightspeed web server and should not be used. right?
Which editor are you using? Images inserted with the default block editor should all have srcset and sizes attributes by default. For other images on your site outside of post and page content, the appearance of srcset and sizes attributes depend on the theme or plugin responsible for the image output. For example, attributes of featured images depend upon your theme.
Themes and plugins can make use of wp_image_add_srcset_and_sizes() to easily add desired attributes. If your images lack these attributes, I recommend asking through the dedicated support channel of the theme or plugin responsible for the image’s output.
There are plugins that can optimize images. They are mainly to convert JPEG and PNG images to WebP and AVIF, but they likely also include srcset and sizes attributes as part of what they do.
You should only use one caching scheme on any given site. Multiple caching schemes can work against each other and result in reduced performance.
Thank you for your explanation
For the second question, I want to choose and install one of these two plugins. I wanted to make sure that I choose one of these two plugins, or should I choose based on the site’s web server?
Why not try out both and see which you like better? Give each several days to let it adapt to your site’s traffic, then run a page speed test using each plugin in turn. Performance shouldn’t be the only criteria. Also consider supporting features, available support from the devs, regular updates to take advantage of the latest tech, security patches, etc.
