Optimise your images
If you have an image-heavy site, like a blog or a news site, the biggest part of your page content transfer is the images. You are probably used to just taking an image, upload it with Joomla's Media Manager, maybe resize or crop it a bit — Joomla 4's Media Manager can now do that! — and call it a day. Good for you, but your images are very likely unoptimised and larger (in Kilobytes, not necessarily dimensions) than they should be.