No, Xbox does not use Mbps.
There are a few potential reasons why your Xbox may be downloading content slowly. First, your Xbox may be experiencing congestion on the network. This can happen when there is too much traffic on the network or when your Xbox is trying to download content from a location with poor internet connectivity. Second, your Xbox might be struggling to keep up with the speed of the content being downloaded.
Yes, 300 Mbps is fast. However, keep in mind that the speed you receive will vary depending on your location, the type of internet connection you have, and other factors.