Here's my show ver IOS version:
Cisco IOS Software, C2600 Software (C2600-ENTBASE-M), Version 12.3(11)T2, RELEASE SOFTWARE (fc1)
It's a Cisco 2621 router providing NAT between fa0/0 and fa0/1. The WAN connection is tested from a device alongside this router as being approx 40mbit. A speedtest of a device accessing the WAN via this NAT router shows almost exactly 20mbit.
Can anyone suggest what might be limiting throughput here?