Internet speeds slower than on Win10
I have a situation I need help on. I have Optimum/Cablevision as my ISP with Ultra300 for internet speed. On my Late 2009 27"iMac (macOS High Sierra 10.13.6) I consistently got 120-130 download speeds. Optimum came out and tested the cable and modem and it clocked at 500 download. So that pretty much ruled out Optimum fault. My work laptop is on Win10 and when I run speedtest, I get closer to 300 (260 - 275) on download speeds.
This rules out router and cable modem issues. All these tests are run over Ethernet connection (I am not looking for advice related to Wi-Fi speeds). Link speed is 1Gbit/sec:
I tried to change the MTU settings in the Network Settings to Jumbo 9000 - this seems to have helped a little bit; now my iMac shows 175-180 on download speeds.
But it is still nowhere near the speed that Win10 unit shows.
Question: Is there anything else I can do to tweak macOS settings to allow higher internet speed, since it is obviously something within macOS?
iMac, OS X El Capitan (10.11.1)