This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
The time required to check signatures and add keys to your public key
ring tends to grow as the square of the size of your existing public
key ring. This can reach extreme proportions.
Gary Edstrom remarked (a long time ago):
I just recently added the entire 850KB public key ring form one of the
key servers to my local public key ring. Even on my 66MHz 486 system,
the process took over 10 hours.