After many more tests I have not found the issue but eliminated a few other problems.
1. The PCI slot in the machine was limiting the danwith to 10 MB
2. The patch cables did degrade the performance. Mybe they should be changed ever 15 years
3Excluded the in wall wiring by using a direct cat6 cable.
When I connect to a local ressource, my synology I get 50/60MB copy speed but when I connect the dsl router i am down to 15MB
For now i have changed my network by using the dsl box as gateway and now my download speed is 30mb as expected.
Any suggestions on what would cause me to lose 50% of my download perfromance ? Upload both configurations have the same performance