Trying to find the origin of strange decoding of binary files

[Nordlöw]

I am trying to find the source of very annoying delay I get when I,
often accidentally, open binary files in Emacs. It does not occur in
Vanilla Emacs so it has to do with some extension I added long ago.
When I open a binary file Emacs hangs for at least half a minute on my
fast computer (AMD64 Athlon X2 4200+), becomes extremely slow and the
resulting buffer often contains lots of chinese letters. How can I
prevent this from happening? Where should I start to search? What
variables and structures control how the character encoding is guessed
and how characters are decoded?

As my Emacs config is huge I thought I would ask for help here first
before I do a binary-searched activate/deactivate in my .emacs which
is very time-consuming.

Many thanks in advance,
Nordlöw