"maybe my corporate IT dept. has borked their standard image"
Not exactly, but it is they're fault. Here's the scoop...
1) The entire Notes 8.x codestream is now an Eclipse RCP application.
2) Eclipse is incredibly powerful and flexible. It's also a huge collection of JAR files that have to be unzipped to run.
3) You anti-virus software is probably configured to scan every one of those JARs every time they're accessed.
4) Like every Windows machine on the planet, your drive is probably also highly fragmented. Because the Notes client is quite bulky in terms of individual files for execution (about 7,000 Features & Plugins at last count) it benefits quite a bit from defragmentation. Try it over the weekend.
Mr. Haynes is quite right that the Linux client is blazing fast. So is the Mac client, especially the new 8.5.1 version that is about to ship. (I'm on the beta.) It's Windows that truly suffers from the many-small-files modularity that the client now has.