I use the pictures in slider in 1920x550 resolution, which I think is correct. I get a bad speed test and lighthouse test witch state that the images aren’t ‘Properly sized images’. An example of one of the pics ( https://i1.wp.com/www.caersbart.be/wp-content/uploads/2019/09/New-Project3.jpg?fit=1920%2C550&ssl=1&resize=1920%2C550 )
Also the slider makes the website shift (CLS), a way to work around this?
The problems are mainly for mobile visitors… the score there is really bad.