Well it would appear if your InfoPath solution works fine otherwise that the issue is with the card and not InfoPath or your form. I do not think you should be the fall guy on this one.
How do you use InfoPath, is this a local template or is it getting pulled over the network from an extenal location?
Are you using webservices or dataconnections within the form that have to connect to external sources?
I am assuming you have verified this on multiple computers with multiple network setups.
My first guess would be some performance issue with these cards or their associated software, either they are taking up more resources on the laptops and InfoPath is not getting what it needs or InfoPath is dependent on the network connection and it is not performing correctly and therefore InfoPath is having issues displaying the data or is waiting for it.
I would watch the performance monitor as you do your testing.
I would also create some simpler forms maybe. One that is just a local form without any external dataconnections and see how it performs. And then maybe a stripped down version of your current form to try an isolate what the issue is.
It sounds like you are stuck with the Dell Cards so I guess using alternative cards is not an option. If you are using the EVDO, I would verify the troughput and latency you are getting, make sure you have the latest drivers, and also look at the client software. Often the default software is more than you need and may be effecting your performance.
Give us more details and we can try to help further.