Help column gets very slow with many scrivenings

All the other top menu items load instantly but clicking on help takes about 4-5 seconds.

You must be running Snow Leopard. That’s Apple’s “helpful” search tool that you can see in the top of the menu. Every time you click on Help, it runs through every single menu item and creates a search index so you can type in menu names, if you are unsure of where they are. Hence it gets slow, because projects with thousands of Scrivenings have lots of menu items (see Go To, QuickReference, Scrivener Link, etc—where the entire Binder is available as menu items).

Actually it was introduced in Leopard… Yeah, it’s an Apple bug. I filed an enhancement request with Apple over two years ago asking if we could have it so that developers can tell the Help menu not to run through certain menu items, and one of the Apple engineers seemed positive, but it never got implemented. It means that when you click on that menu, as Ioa says, the Apple Help system runs through every single menu item, building every single menu in the process - for every menu that lists all documents (Move To, Go To, Scrivener Link etc) that can be slow-ish. It’s annoying, but out of my hands until Apple give developers a little more control over what should and shouldn’t be indexed, unfortunately.
Best,
Keith