This is what's going on behind the scenes:
You are copying 6430 files and after some time the copy slows down.
As you're seeing at that time python files being copied, the natural
conclusion is the python files are slow to copy.
It's possible that the problem is with your antivirus, that insists
on checking the files one-by-one, but this is somewhat less likely.
I believe that this behavior is because what you're actually seeing
is how fast do the caches fill-up on Windows and in the target
(if not on the same computer).
Windows will start by reading from the disk at its speed
to copy it to the target at a lesser speed.
At this stage the speed you're seeing is basically that of reading
from the disk.
When the Windows memory cache fills up, the speed will drop to that of
writing to the target, since cache memory needs to be liberated in order
to read more from the disk.
The speed then drops to the writing speed, which is always less than
reading speed, especially for small files.
In addition, you're using Explorer for the copy, which is one of the
slowest products I know in this domain.