Hello,
I have a mikrotik RB750GL with the last RouterOs 6.37, 1Gb speed on each port, and with a different network range on each port, but when I want to copy a file from 192.168.10.xx to 10.10.10.xx, the transfer speed Is 6 MB/s very slow.
I dont have any Qos rule, just a NAT rule. 
I appreciate your help
If you don’t need any firewall rules between internal networks, do no track the connections between them. Otherwise try fasttrack.
In fact I have a few firewall rules for filtering external attacks, and nat redirecting for a cache server.
Can you explain a little bit about using Fasttrack
My answer was not related to rules against wan. Read it again.
See this. http://wiki.mikrotik.com/index.php?title=Manual:IP/Fasttrack&redirect=no