Imagine you want to edit a documentary film in Blender and you have hundreds of 4K video files. Generating multiple proxies at once is possible now in Blender but challenging, you cannot import 200 files in the VSE and ask Blender to generate proxies, if you do so it will crash. You can only do it little by little and it is very time consuming. The best solution would be to generate proxy files outside of Blender in a batch way and back in Blender link all the high res files to the low res files. The proxy custom directory is made for this but one needs to click on "rebuild proxy" I understand? When you click on "rebuild proxy", although you've already generated proxies, Blender will take ages to connect the high res files in the VSE to the proxies you've already generated. As if Blender is doing the work again. Am I doing something wrong? Thanks.
you shouldn't need to rebuild the proxies if you have already made them - so long as they are the right files in the right places, just set the paths, and you should be able to switch to proxy viewing modes.