Major slowdown in set-buffer-multibyte

[Stefan Monnier]

I recently tried to open a zip file (6MB, nothing enormous), and it took
me a while (1minute).  Now, admittedly, I have all kinds of assertions
and de-optimizations enabled, so I recompiled the trunk normally to
check it wasn't my own fault, and the problem is still present.
ELP tells me that set-buffer-multibyte is called twice and that the two
calls take a total of 9s, with Emacs-22 opening the file is instantaneous.

By hitting C-g a few times it seems that the problem is in the
(set-buffer-multibyte 'to) call (i.e. converting from unibyte to
singlebyte rather than the other way around).

Could someone look into it?


        Stefan