I got (another) comment on IRC about how guestfish is slow. Please try the latest version. It’s far faster than the ancient libguestfs 1.2 that you might have tried 6 months ago in RHEL 5 / RHEL 6.0* / Fedora 12.
By a factor of ×4 or ×5.
Here’s guestfish starting up (with a hot cache) on my laptop which is four years old:
$ time guestfish -a /dev/null run real 0m6.403s user 0m1.538s sys 0m4.595s
*If you are using RHEL 6.0, please try the RHEL 6.1 preview packages which include all the latest performance improvements.