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If the CPU is not the bottleneck, there is something at your provider (probably throttling Tor traffic to balance the other non-tor users in the same datacenter). If you built the network infrastructure there and know for sure such thing is not implemented there, don't really know what to say. CPU / RAM and Network interface is all you can test to see if it is the bottleneck for Tor. If all these are off the list, there is something upstream you.
I recall reading on the Tor blog that there is a bit of a lull after you get the Guard flag for your relay. Given that these days Tor clients only choose one guard, is it possible that his relay just hasn't built up a list of Tor clients using it as a guard yet?
Thank you, Derric Atzrott