I've not tried the Max one but the C4D VFB is quite slow when you get to large frame sizes, especially with bloom and glare still. I've had a couple of B&G no shows on the latest beta this week even with tweaking the VFB window to refresh. Fortunately I could save to CIE and get a lightmix pass with bloom out to comp back in with the rest of the multipass in PS. Is having to sync with the PV an issue perhaps? Im still hopeing to get native 16bit PSD as an output from the VFB. I still struggle to work out why the VFB would be different in Max as it would seem like a 99% pure corona controlled environment. But like Nkilar says, Im excited about the develpment too.
Are you talking about the refresh until PostProcess-Settings are applied? Or are you referring to high response times in PV (e.g. lagging while panning, etc.)
The latter might be caused by a custom monitor profile which is set in preferences. If it differs from sRGB, PV will slow down.
edit: Misread your post. You're talking about VFB, not PV. Yes, it is slower than Max.