Slow speed with both DD-WRT and Tomato

Hello,

I recently purchased a netgear R7000 and did a few tests with it. I noticed when using either DD-WRT or Tomato firmware I was much slower than when using the default firmware. Here are the results I got :

LAN Speed :

My current setup is the following : I have a PC and a NAS. Both are connected to the router using cat5e for gigabyte speed. With the default firmware, I get about ~110 MB/s which is about what gigabyte is capable of. When using DD-WRT or Tomato (With CTF enabled), the transfer speed slow down to about 10 to 20 MB/s. I'd like to know if it is possible to have gigabyte speed other than with the default firmware.

WAN Speed (with VPN) :

I pay for a 50 down / 50 up mbits/s connexion. When tested on speedtest.net with the default firmware, I get about 63/59 mbits/s on average. With DD-WRT or Tomato I get about the same. The problem comes when I am turning on the OpenVPN client. My speed result then drop to about 19/37 mbits/s. When testing using the windows client instead of the router, I get 60/57 mbits/s on average, or a drop of about 5%, so I know that the server is not faulty. I was told by the vpn company tech support that the router's cpu is usually the bottleneck in that case.

The maximum CPU usage I got when testing the speed was about 26% for 19 mbits/s download speed and 52% for 37 mbits/s upload. The data seems to indicate a linear pattern, and it could be said that the maximum speed I could get before reaching 100% CPU is ~75 mbits/s, which is much faster than my internet connexion. What I'd like to know is why the download speed is much lower than the upload speed, and why it is overall much slower than the PC application when I know the CPU can take it.

Thank you very much for your help !