Slow speed test results - but not direct from modem

Hi all,

I have noticed slow speeds on my newish Slingshot Gigabit connection that became noticeable recently when trying to watch Twitch streams on the higher quality settings (1080p).

The speed tests I'm getting are quite low - usually between 90 and 300 Mbps

http://vocus.speedtestcustom.com/result/18fcdf00-eefe-11e9-928d-87ca243f4f62

http://vocus.speedtestcustom.com/result/49e9f6e0-f05f-11e9-a337-cfd7ac5e0c44

When I emailed the helpdesk they pointed out that when they run their diagnostic test directly from my modem, it shows speeds over 900Mbps. (I confirmed this by running the test from within the modem interface myself).

Can anyone explain why I might be getting reduced speeds on my devices. The two tests linked above were done using two different ethernet cables, and on two different PCs.