SiteA> Site B > Site C
Site A to SIte B - Bandwidth test - 40Mbps
Site B to Site C - Bandwidth test - 50Mbps
Site A to Site C - Bandwidth test - 10Mbps
The MT routers are doing OSFP and every site connected to each other using Ubiquiti PoweBeams.
MTU in each device is 1514. IP network and osfp are running on VLAN (1514MTU).
What could be wrong with speeds between Site A and Site C.
It seems that MT at site B is busy handling forwarded packets, so you firewall rules in chain=filter may be so complex that the CPU doesn’t cope with the volume of the traffic.
Or there may be some QoS rules in place which affect the transit traffic on either of the three Mikrotiks in question.
So while the A<->C bandwidth test is running, check what /tool profile shows you on Mikrotik B, and whether no kind of queues is activated anywhere.