Not a “bug”, but more a configuration issue I would have to believe.
The reason is for example, even on the WRT3200ACM, which has the most powerful of the Marvell CPU’s currently, it is only able to right roughly 100Mbps over OpenVPN, with HW acceleration.
So when you are saying you get 10x that on a very similar CPU, it is very hard to believe.
Heck, the Netgear R9000 quad core Alpine CPU is only in the 150Mbps range…