Why is the option "Process in the front-end" gone?
As of version 5.0, the "Process in the front-end" option is no longer available. This option allowed you to automatically optimize images added by users, for example from a form they filled out. At the same time, it triggered the normal optimization processes for websites where administrators were not always logging in (in such cases, the optimization was not triggered automatically).
We decided to remove this option due to performance issues. For busy websites, it could easily put the entire website down, especially on servers with very few available resources, as it triggered a call to the backend for every click on the website to check if something needed to be processed by ShortPixel.
Starting with version 5.0, there are WP-CLI commands available as well as adding cron jobs using WordPress plugins, that allow you to achieve the same result as the "Process in the front-end" option by running WP-CLI commands via regular cron jobs.