Documentation: update performance.txt
Documentation: update performance.txt

Bisecting shows that the performance drop is caused by
this commit:

commit ca9e912a28b901387e1dbb85f6c531119f2d5ef2 (refs/bisect/bad)
Author: Jakob Unterwurzacher <jakobunt@gmail.com>
Date:   Sat Feb 29 19:58:08 2020 +0100

    fusefrontend: drop xattr user namespace restriction

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs