So I have a mid-2010 mini mac w / 8gb RAM that I mainly use as a media server and some download. I made a new installation of Mountain Lion and I got a download of 300 Mbps, but some programs require at least El Capitan, so I have updated. But after the update, my download speed is very unstable during the speed test and never exceeds 120 Mbps. I thought it was perhaps a bone problem, so I tried Sierra and High Sierra with the same results.
My advanced hardware parameters are Speed: 1000baseT Duplex: Full duplex, MTU flow control: 1500
I have tried the ethernet cable in 2 other devices and both get over 300, there are no updates for my router (R7000p) and i also restored the router in factory. Performing a speed test only on the Wi-Fi network ends with very similar results with lower speeds
My knowledge of macs is limited, any help would be welcome.