Is it less like lagging and more like hitching or jerking?
I use WPFInterop as well and hitching is something I've just come to accept on lower-spec hardware (also using an Intel HD4000 as min-spec target). Since D3D11 has to go through D3D9 there's a crapload of Locking involved.
On my low-end machine each of those locks is ~5ms, which doesn't leave a whole lot left for both WPF and my rendering to work with. I've been able to dodge it since I'm using MG in WPF for a viewport in a tool so often the viewport is fairly small (~20% of window space).
I've come to the point of mashing WPFLight and Winforms MetroFramework together for version 2.0 to dodge the whole D3D11-in-D3D9 mess.