Imaginary is a small addon that will help with this. It will resize on the fly and it is pretty quick
Previews is used due to problems to serve large images. One is scaling in browsers one is timeouts on slow connection. One is that if nc needs to rescale an image it uses cpu.
Many photo gallery apps use (web apps) uses preview generation to gain the ability to show things faster. And yes you will loose some space.
It seems like you have a combination of issues where some media is federated and takes time and some is local and previews of many small files eats connections and cpu at the same time.