In other words, they were going much faster than was routine to approach a checkpoint manned by armed soldiers with orders to defend themselves as they needed to.
I was struck by that quote, too. She didn't say "we weren't going fast at all" or "we were going rather slowly"... she said "we weren't going very fast, considering". That tells me they were going pretty fast (keeping in mind that she has reason to downplay their speed). Also the reports are that it was a windswept, rainy night. Any car approaching a checkpoint near a U.S. base, on that dangerous stretch of the Airport Road, in those conditions at anything but a cautious crawl is courting danger.