Hi, I'm trying to efficiently get a list of all files in a category with 1'000'000+ files. For example all files on Commons in this Category "Category:Flickr images reviewed by FlickreviewR 2". This was the most simple I could come up with, but it takes like forever to get an output: https://quarry.wmcloud.org/query/83527

Is there a better way or does this require another tool (which would that be)?

You would have to write a dedicated tool to do requests that divides the query into multiple chunks and writes them to a file. You could use the Special:MyLanguage/API:Categorymembers api for instance, and list per 500 (limit) and use the continue parameter from request to request. Or get a database dump installed locally and export them from there. A million is a lot, it is not a type of request that the wikimedia optimises for.

Also depending on what you are trying to achieve, there might be alternate/better ways to achieve that goal other than listing a million+ entries in one go.