Vista - Incredible slowdown with Vista UAC

We have a .NET/C# application that is required to start automatically
as soon as an user logs on to their PC. It is well known that Vista
blocks applications that requires admin privileges (ours does and it
specifies that via the manifest) to automatically start on logon.

Our problem is looking far more serious. We launch our admin-
demanding app via a batch file and put _that_ batch file on startup.
This works in that the app now starts fine at startup but it really
really really really slows down the application to a crawl. However
if I back out of the admin privilege and publish the app with a
manifest with just asInvoker, everything is just fine and dandy. The
app launches fine on startup and functions equally well.

Why in the God's name would running an admin privilege seeking
application at startup cause so much slowdown?

Is there some Vista document I am missing?