The windows update system is always like that, it is one of the reasons it is moving to a new update format system. also windows will pause download updates if it notices free system resources getting low.
I think it depends on demand and number of servers available at the time to download the updates. I noticed last year it only goes at about 12mbit when downloading from the store, none of the Microsoft server connections are amazingly fast but that could be my local servers and as the average speed of connections where I live is lower than 12mbit it may not be worth their time having a faster connection speed.
The windows update system is always like that, it is one of the reasons it is moving to a new update format system. also windows will pause download updates if it notices free system resources getting low.