It is hard to say you get slow speed, Normally there should not be any reason for that. I got full speed myself when coping between my two ssds.
But reasons for slow down can be so incredible many and so much is affecting the speed.
It can also depends on what settings you tweaked.
For example some drives that have a very large cache, then make sure drive cache is enabled for write.
Also some OS version works better with some buffer size settings then others.
I assume that you start the copy so that MC is doing the copy and not Windows ?
(The MC copy progress dialog is shown and not the one that Windows Explorer shows ? )
In the file operation log you can see if it is the read or if the write that is slow or if it is both.( since each time/speed is logged there separately.)
When you know what part that slows it does then you have a bit more to go on.