We experienced this exact problem when we converted our system to ACS Classic 4.2 just over a year ago. The problem stems from the fact that, as you say, the page takes a long time to download. What's worse, the page that we were using presented the list of users in a drop down box. Now how is it that a drop down box makes
any sense whatsoever when you're talking even a small number of items by comparison (say even 100 or so).
In the end, I ended up slightly modifying and inserting a new page into the process of granting permission. The page that I added allows the administrator to enter in text to search on from the list that gets generated. I believe it only allows searching in the email address the way I wrote it, but it could be extended to included name as well. Anyway, we've found it to be invaluable in granting permissions.
As a side note, the permission system in ACS Classic 4.2 just sucks, but that's just my personal (not so humble) opinion.
If anyone is interested I would be happy to go back and try and disect the changes that I made and make them available.
