One of our networked computers was likewise incredibly slow on 4.61 - getting to the point it wouldn't really open at all unless we were remarkably patient. The problem remained after upgrading to 4.63.
The above-mentioned Internet Tool setting has always been on, and I tried options such as "-roam" and much else besides. Ordinary network traffic was consistently fine to that computer, and Pegasus on all other computers was ok.
In the end, I solved it by deleting Pmail.cfg and running PMConfig to recreate it, very easy. This definitely fixed the problem, but I made one other change in conjunction - instead of using a dedicated share I'd had for the past two years (\\server\pmail), I reverted to using i:\pmail (via mapped drive I), which we'd previously used without problems for a decade.
One or other of the above steps fixed it for us.