I have two CRS312 in operation at different sites/configs and have noticed similar issues.
CRS312 connected to different machines in 10GB + One connection to a router in 1GB.
Connections in 10GB between machines on the CRS312 are fine and fast (tested with iperf).
Connections between machines on the switch and the router or machines connected to the router are slow and unstable (tested with iperf).
On one site, the router is also a mikrotik (rb1100x4), so i used the Bandwidth test feature .
i'm getting full 1GBit/s in send from router to switch, more or less 500Mbit/s from switch to router. And when traffic is both ways switch to router is all over the place and slow.
I tried setting traffic flow control to on and off on the interface from switch to router, but same issue.