making slider smaller and centered without jumping to improve seo

BTw; i restored the website with the normal slider because of this ‘jumping’

see test domain;

https://sub.kunstkapers.nl/

What if instead you set the container width to that width?
So removing your code and adding just:
#customizr-slider {width: 1050px;}

and of course fine tune it for different viewports.

Anyway FYI you’re using images of 1170px x 400px or even 1000px x 400px (the first image). So changing just the css will not let you improve the page speed.
Anyway having images with a width of 1050px instead of 1170px don’t think is this great improvement, but your choice.
To use, then, images of 1050px x 400px you should add the following to your child-theme functions.php and regenerate your thumbnails (regenerate thumnbnails plugin):

add_filter( 'tc_slider_size', 'my_slider_size');
function my_slider_size() {
    $sizeinfo = array( 'width' => 1050 , 'height' => 400, 'crop' => false );
    return $sizeinfo;
}

…Anyway more than the slider’s images I’ll take a look at the sliders in your footer.. ’cause it’s there where you display small images while you retrieve bigger images.
There you show images 190×150 or 105×150 or 200×150 and so on, but the actual loaded images are far bigger. There is the real problem.
Make the speed test disabling those widgets and you’ll see.

Cheers

Thanks ‘d4z..’ ?? The first part of changing container width and fine tuning for different viewports sounds a little bit difficult to me but i’ll give it a try! But your second tip of using smaller images for the slideshows on the bottom allready gave me a far bigger improvement than i expected!

Maybe it’s also possible to add a widget on the side, what i allready had in mind, thereby forcing the slideshow to be smaller?