Gig ethernet through fast ethernet port

One of our customers recently brought in an ISP offering 1gpbs internet but we're only seeing speeds of about 300mpbs. That's a little puzzling to me because of how the physical and logical network is configured. They have an MPLS line going to a Cisco 881 router with 10/100 ports, connected to that is the gigabit switch stack. Connected to the switch stack is a Cisco ASA with gig ports and the gig ISP is connected to the ASA. The switches are only layer 2 and they have the default gateway set to the 881, so all internet traffic is going to the 881 first, then switched back though the ASA for any non-business traffic.
I would believe that should limit traffic to 100mpbs max but I'm somehow getting 300-400mpbs on different speed test sites?