One of the most disturbing WTFs ever. I personally found this comment almost as bad:

Of course a DBMS is the right tool for this job, but I don't think you can lay the problems of this system on the choice
of flat files in a directory tree.

You can go pretty far with a directory tree (it is a tree index, after all) as long as you keep the size of the directories small.
For example, if user id #0123456789 was kept in the file

01/23/45/67/0123456789

Wasteful of space, but not horribly slow. Subsecond access should be no problem at all. And you could distribute this system nicely by user id, eg the last digit of the id is the backend server number, 0-9, to send the scan to, so there need be no problems with immediately debiting the user account.

Sounds like the performance problem was due to the muddleware.

Yeah, you could. But there were better solutions in the early 1960s, and it's been over 40 years since then, so there's really no point in spending lots of time screwing around with such ideas. Ouch.