asymmetric performance for network I/O

Hello,
I did some tsts with KVM virtual machines.
I gan good results on gigabit interface with inbound traffic, I get almost gigabit saturation (1Gb/s). I have lower perforance outbound that is around 500Mbit/s.

Is it normal in your opinion ?
I mean is there some hard coded limit for network I/O performance ?

any experience of someone else about this topic ?

thanks

Rick