No responsive images sizes in slideshow module?

Hi there! A customer is using a lot of slideshows on their landing page and eventually wondered, why it takes so long to load. Apparently the images chosen to show in those slideshows (lets say 3 images per slideshow, original image is 2200px width) are being displayed in their original size, scaled down obviously, but the original slideshows are in a grid which is roughly 300px width for each column, so the original images are way too big. Is there a way to bulk-scale them down, like override an image class or something to create multiple corresponding image sizes for the slideshow?

Cheers, M