Linksys Wireless Router :: WTR320N Still Slow After Upgrade

Today I upgraded my 10/100mbps LAN to 100/1000mbps LAN with Wireless using the Linksys WTR320N. The initial testing went without problems. The small LAN I setup with 2 laptops, router and modem worked fine and I plugged in the other systems.
 
During the testing the speed of the internet was like before, instant loading of most of the pages (i.e. Facebook, Youtube) and reached the max speed capable, which is 23MBps together with a low ping of arround 30.
 
Now, with the full network the internetspeed doesn't top 20MBps anymore, it sticks back to 15-17, which is a loss of 6MBps on my connection. After checking out various topics on this and other forums I upgraded the firmware from 1.0.0.3 to 1.0.0.4. Which gave a small improvement but I still don't reach my max speed, nor a decent ping and pageload times.
 
I got a Cable-internet provider which doesn't require password/username to logon to, so it's a permanent connection. MTU is at 1500, and because I don't know what it is (I saw the phrase and changing it to 1300/1350 a few times while searching for a solution).
 
How I can speed up the loadtimes and internetspeed? The problems are both on wired as wireless connection. Signal strength of the wireless is maximum and the wired connections never gave problems before.