What does very slow and not stable mean? How are you determine the My Cloud is slow? Have you run speed tests from the computer to the My Cloud to determine the read/write speed? By not stable what is happening to the My Cloud? Is it disconnecting from the network? If so have you checked the router the My Cloud is connected to?
Is the My Cloud connected to a Gigabit router? Is the Macbook connected to that same Gigabit router?
Have you checked to ensure the Macbook is actually using Ethernet and not wireless? Sometimes one has to disable the wireless adapter on the computer to force the computer to use the Ethernet connection.
A lot depends on the setup of your local network including the capabilities of the network router, any network switches used between the router and client devices. Using WiFi can cause slow downs. Using older network cabling not rated for Gigabit speed can also cause issues.