It's not that I don't believe you, it's just that I think you're looking at the wrong thing; you want to test the speed of your intenal network, not your external network, which is what Speedtest is doing; it's going to an external test host server.
Your PC is connected to port 2 of your router, isn't it?
You have a 100Mbps WAN (tallies with your Speedtest result).
You have a 1gigabit link to your MyCloud (tallies with the green network LED).
You have a 100Mbps connection to your PC. You need to find out why that is, and correct it.
Are you sure you have a Cat5_e_ cable?
In Windows, start the 'Network and Sharing Center'
For your 'active networks', select 'Local Area Connection'.
That will show the link rate, and allow you to set the properties of the interface, and diagnose faults.