[jcranmer]

Waste isn't the issue with big folders; filesystem performance (especially NTFS, which is to say Windows users, which is to say ~90% of users) degrades pretty horribly when folders have tens of thousands of files. Even on Linux filesystems, having a folder with a 100,000 files in it was having issues.

[kevincox]

FWIW on ext4 I benchmarked adding millions of files to a folder and compared with `xx/...` and `x/x/x/x/...` and found the performance to be basically identical between the options and didn't degrade with the file count.

The one thing that I did find is that deleting folders with many files was very slow. I guess that path wasn't well optimized. However even then you could deleted hundreds of files a second so for regular mail usage it likely isn't a major issue.