Pull up the log in page for the hub. But don't log in, just click on the link "Check router status" That'll bring up a window with five tabs. Open the Downstream tab. Select all the text (Ctrl-A if using a keyboard), copy it (Ctrl-C), then paste it (Ctrl-V) into a reply here as TEXT not screenshots. Post that, do the same for the Upstream and Network log. You'll get an error message when you post the Network log, just click on "post" a second time. Then we can check for any obvious problems with power, noise or error counts.
to add to the above - try a test with the hub in modem mode if you get the same result put the pc - assuming its a windows pc into safe mode with networking - again with the hub in modem mode and run speed tests
finally try another lan cable - cat5e or better - 90 can be a faulty cable although the fact you are seeing 300 before it slows down would suggest its not the cable but stranger things have happened