If you are facing slow speed issue only on the direct connection we would suggest you to use different ethernet cable of cat6 or 7 and try checking speed on the device by connecting it to different ethernet port on the modem and if still same issue persist we would suggest you to check the cable signal level on the modem interface by following the steps mentioned on the below article. So here we will be checking the signal comes from out to inside the home. There are some important things that should be within a specific range in order for the internet to work properly.

First, the SNR should be above 33 on all of them. The downstream power should always be between -15 and +15. The closer to 0 the better connection. Finally, your upstream power should always be between +45 and +51

On all of those values you should not have more than 3 decibels (or dB) between the lowest and the highest

By having the values in less than that, your device will have interrupted connection, slow or no connection at all.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Cable-Signal-Levels