Theoretical performance hit via Voxalot

[Tranworld]

I am new to this service and found it amazingly useful, at least when registering my SMCWTVG to Voxalot it always gives good dial tone when otherwise it's hit and miss when using other direct VSP (we won't dwelve into the why's here, I think this is a bug in the device, the Linksys ATA don't have such problem), so I am going to start using Voxalot exclusively for this device.
I have a question as to whether there should be a performance degradation, even if it's entirely therectical, when going via a 3rd party registration such as Voxalot? May be an additional packet delay? Also what appens to the STUN server configured on the device, does it still provide a straight voice stream between the two connecting devices? (thereby negating any real performance degradation)