In such cases, where the uplink (300 Mbps) is slower than the speed of the ethernet (ports) in the LAN (1 Gbps) the solution might actually be to turn on flow control on all the MikroTik router’s ethernet ports on the affected path. Flow control is per default off on MikroTik devices so you can try to turn that on.
Here are some recent examples where doing so helps when traffic from a link with higher throughput rate need to be squeezed into a slower link:
http://forum.mikrotik.com/t/ccr2004-1g-12s-2xs-slow-speeds-compared-to-old-crs125/182775/1
http://forum.mikrotik.com/t/bad-performance-with-crs310-8g-2s-in-working-at-2-5gbps/182429/1
I also provided some explanation in the 2nd link that might apply to your situation.