|When users take the laptops offsite & connect to a wireless network, they experience delays up to 10 minutes when they try to login|
I suspect the issue is the mobile units are trying to authenticate to the DC (ie: login to your domain) and that's why the long pause. Unless they're connecting to a part of your domain with network access to your DC, this isn't going to change unless you change how they login.
Try giving them a separate Local user account on the laptop itself that doesn't connect to the domain. Then, when the user is out of office, they select that profile from the "Log on to" pulldown menu. When at the office, they select the one showing the domain.
Test this one one laptop first to iron out any issues before deploying on all laptops. Oh, and I wouldn't make those Local Administrator level accounts unless you want to give the users the ability to make changes and install software.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.