This test shows what happens when both the wireless and routing sections of the router are running full tilt. The setup is the same as shown in Figure 1, but this time I ran only one downlink stream each on the 2.4 and 5 GHz radios and added a single WAN to LAN wired stream.
I first ran the tests with no bandwidth limits, i.e. blasting each stream as fast as it would go, which produced the results shown in Figure 5. The test starts traffic on the two wireless radios first, then the WAN to LAN routing traffic. You can see the dip in wireless throughput when the routing traffic starts and a huge rise in routing throughput when the wireless traffic stops.
Figure 5: Simultaneous 2.4 and 5 GHz plus routing - full bandwidth
Of course, not many of us, at least in the US, have 200 Mbps download speeds. So I limited the routing traffic to 50 Mbps for the test shown in Figure 6.
Figure 6: Simultaneous 2.4 and 5 GHz plus routing - 50 Mbps WAN to LAN traffic limit
You can still see a reduction in 5 Mbps throughput when the routing traffic starts at the 10 second mark. But there doesn't seem to be much effect on 2.4 GHz throughput.
You would probably only see these effects if you had a very fast Internet connection loaded with heavy download traffic while running heavy streaming traffic on either or both wireless radios.
Of all the effects shown, the throughput reduction when both radios are simultaneously used is the one most likely to bite you in the butt, especially for uncompressed high def video streaming.
These further tests show that the E4200's definition of "Maximum Performance" doesn't necessarily mean maximum on both radios and routing at the same time. Perhaps there's a Simultaneous Maximum Performance router from Cisco somewhere in the product pipeline.