Bad download speed from lan switch to wan on WRT3200ACM


I have at home a Fiber/Coax internet connection 400mbit/40mbit (Down/Up) from SFR in France.
I put my WRT3200ACM behind the modem directly with DHCP on Wan side, getting a public IP.
I used Iperf to test the bandwidth directly from the router, and I get pretty good numbers 350mb/38mb.
Then I tested from a PC wired to the switch and I got pretty bad numbers for download (5mb/38mb).

I am new to OpenWRT I just got my router using the latest stable build. I started to investigate, work on MTUs, MSS Clamping... But no real success.
Then I started to play with the switch config. I did not understand why the default configuration was configuring the CPU ports in trunk mode. Since there are 2 CPUs there was no need to set vlans on the ports.

This configuration changed completely the results, now I get pretty much the same results from the LAN or directly from the Router itself. So it is great by I don't really understand why :frowning:

Also I setup an IPIP tunnel towards another machine on the WAN side. I tried to evaluate the bandwidth of the tunnel. When I use Iperf from the router targeting the other end of the tunnel using its internal IP (ensuring I really go thru the tunnel), I get great performance, pretty much the maximum bandwidth (350/38) but when I do the same test from a PC on the Lan I get "only" 150mb/40mb.
Also looking at the CPU load on the router, everything looks fine, less than 30% load.

Did anyone experience the same kind of issue? is there a known issue with the switch?