Hi,
I'm Rodrigo and I'll help you.
The system Data usage is probably high because it is probably downloading some updates, after completing it, it should normalize.
About your download speed, are you seeing this download speed in Xbox app, Steam or Epic?
If yes to one of these platforms above, it means that the download speed is correct.
They show the download speed in MB/s (Megabytes per second) and the connection speed you contract (or in the speed test) is shown in Mbps (Megabits per second), these are different values, and if your download speed for a 200 Mbps connection is around 20 MB/s, it is around the normal values.