With the resurgent interest in virtualisation, router virtualisation seems the next step to servers virtualisation. It is also a key in deploying flexible and isolated virtual networks. In this paper, we evaluate Xen-based virtual routers on top of commodity hardware. Forwarding being achieved through the driver domain networking model, we identify and analyze the throughput limitation through profiling the CPU usage and the memory accesses. In multi-core systems, the memory access latencies are then the bottleneck. Then, we propose in this context a mechanism that classifies packets and schedules them so as to guarantee them the maximum throughput and the minimum delay based on the priority they are assigned.