Mikrotik NAT halves WAN speed

The 951G-2HnD has a rated peak routing speed of 250-300Mbps. It does its switching in hardware and should be able to switch at wire speed. So it sounds like you're getting roughly the performance the hardware is capable of.

David's answer is correct. To summarize, the hardware is capable of switching at the speed you got, but only routing at a fraction of that. Therefore, the problem is the routing, not just the NAT, although that probably doesn't help.

Take a look at these results for routers that should be capable of routing that much traffic:

https://mikrotik.com/product/RB750Gr3#fndtn-testresults

https://mikrotik.com/product/RB3011UiAS-RM#fndtn-testresults

Any of these three should be enough unless you're doing a large amount of very small packet routing.

Another possibility is the CHR product - you can run your router in a VM and give it as much CPU and RAM as you need. $30 covers the cost for a 1Gbps cap.

EDIT: Somehow https://mikrotik.com/product/hap_ac2#fndtn-testresults has better results than the 3011 for non-full-size packets despite having significantly less CPU and RAM, and being on the same architecture.