I bridged my ISP router and am currently speed testing my personal router on which I flashed OpenWRT.
The DL speed of my ISP is 400Mbps (tested and validated via the ISP router).
However, after the bridge the speed doesn't go above 270Mbps.
If I transfer files from my PC to my NAS I get good speeds of around 720Mbps. (Ethernet both sides).
I captured some network traffic with wireshark, all seems normal.
I believe the problem comes from the CPU of my personal router. While running DL tests online, the Usage tops at almost 100%
Thank you all for the very prompt replies.
The Software flow offloading option worked for me. Now it's reached about 380Mbps at 77% CPU load. I am going to try to figure out if my MIPS can do hardware offloading as well. @dlakelan I read your post and honestly it's very good. Definitely going to dig deeper into the subject.