I think that's going to need to happen is:

• Removing "you have this item" from the pages when you first load them.
• You click a button that shows all of the "you have this item" items on the wishlist.

That should speed things up a lot.

I've moved the pages to 250.

This has nothing to do with your personal computer speed, it has to do with pulling down hundreds of items from the database and checking if you have that item anywhere on your account. The pages were added at the same time, but will be slowly phased out as we continue testing the wishlists.