Definitely possible with LTE (4G), and very possible with later 3G standards. My now two year old LTE capable phone frequently sees 70Mbit service in downtown Seattle (Fastest test has been 92.88Mbit). The new upgrade I'll pick up later this year will push that higher.

Cellular networks have advanced a lot with the rise of the smartphone. All those "cheap little boxes" are just slimmed down smartphones basically.

Being that the laptop is on XP, I'd fear it's infected with malware that was causing high amounts of traffic. I'd personally never use an XP or older OS connected to the internet at this point.