In "The Art of Unix Programming", on the topic of The Terminfo Database I read:
If you look in the terminfo directory, you'll see subdirectories named by single printable characters. Under each of these are the entries for each terminal type that has a name beginning with that letter. The goal of this organization was to avoid having to do a linear search of a very large directory; under more modern Unix file systems, which represent directories with B-trees or other structures optimized for fast lookup, the subdirectories won't be necessary.
I wonder if there are widespread (i.e. production ready) filesystems with this quality.