Switch – Different port bandwidth on switch causing massive bandwidth loss

i think i am missing some basic knowledge about network connectivity, maybe someone out there could explain me, why the following causes problems. The facts:

• We are connected to the "internet" with 100mbit/s, full duplex.
• We had the cable of our internet provider connected to a security appliance (Cisco ASA): outside- and inside-port where configured to both 100mbit/s, full duplex.
• Behind the security appliance we had a 24-port 10/100/1000mbit/s switch. The port connected to the ASA was configured to be 100mbit/s, full-duplex … the other ports where configured to be 1000mbit/s

Internal bandwidth between the machines connected to the switch was always very good. Incoming bandwidth was always ok. Outgoing bandwidth was ok until after two years suddenly the outgoing bandwidth dropped down to below 1mbit/s.

At first we thought we had a problem with our ASA, because we detected lot's of CRC errors on the outside-port of the appliance. We swapped the hardware, but the bandwidth problem was not solved.

We than changed the configuration of the inside-port of the ASA to 1000mbit/s, full duplex and the port of the switch to 1000mbit/s, too … so that every port of the switch now has 1000mbit/s.

This not only solved our bandwidth problem, it's even better than before. Apparently we had some kind of bandwidth-mismatch because of the different configuration in the switch … but i am not really sure, why … is there some "easy" explanation for this kind of problem?

Thanks in advance!