If you use Django’s file-based cache (where cached data is saved as files on disk instead of in memory or a database), how well does it work when you have a huge number of items stored in it — like 10,000 or more? And at what point does it become noticeably slower?
Ultimately, there’s not going to be a pat answer to this. Too much of this is going to be system-dependent. Your performance curve is going to vary greatly depending upon the operating system, file system, amount of memory available for the file system cache, the nature of the storage device(s), the storage device cache, and any “abstraction layers” that may exist between Django and the physical device. Overall performance will also be influenced by other activity on that same device.
The only valid answer you can rely upon will be what your own testing reveals in your environment, and whether it satisfies the fundamental requirement of “being fast enough”.