As you see from the MC dialog it is the write speed that is slowing down the process.. Read speed is almost 300mb/s, Windows only show you the averge speed of something, probaboly a combination of both the read and write..
Why you got slow write speed is very hard to say. There are so many layers that the data goes via before they are written that can affect speed.
If you expand the 'copy to ' under advanced you got read/write stratergy, there you can tweak buffers and chunksizes and if caches should be used or not.
security software may also cause issue, Some do not like unknown application doing file transfer and will do extra scanning of them.