UUID as primary key

I have a system where it would be very useful for the primary keys for
a few tables to be UUIDs (actually MD5s of files, but UUID seems to be
the best 128-bit type available). What is the expected performance of
using a UUID as a primary key which will have numerous foreign
references to it, versus using a 64-bit int (32-bit isn't big enough)?

From the uuid.c in adt, it looks like a UUID is just stored as 8
consecutive bytes, and are compared using memcmp, whereas an int uses
primitive CPU instructions for comparison. Is that a significant
issue with foreign key performance, or is it mostly just the size that
the key would take in all related tables?