I was installing Crypt::DH, the Perl Diffie-Hellman library today. The tests took 20 minutes on a Macbook Pro. Then I noticed a comment on an OpenID forum about "making sure the GMP Perl bindings were enabled" to speed things up.
Specifically this means install Math::BigInt::GMP, as I found out, after some searching. The same tests ran in less than 10 seconds using the GMP library. That's impressive.