Can you test without the MikroTik? I've been having similar issues with my Mikrotik set up (I had a half a dozen or so posts on reddit about it). It appears the devices are not handling flow control correctly and the buffers are being overwhelmed. I'd test first without the Mikrotik and then experiment with flow control on the port. Be aware that you seem to have to disable and reenable the interface for flow control to take effect (or reboot). It has been getting better with more recent versions of RouterOS but it is still not "right" it used to be much much slower.
Edit: fixed a typo