Shock load from falling belayer(s) when a girth/clove hitch anchor slips under FF2 ?

In a leader FF2 from a hanging belay, how much speed would the hanging belayer (+perhaps a 3rd person in the party) gain while the clove/girth hitch is doing its ~5kN slip ?  And how much force would the belayer (+potentially the 3rd person) then exert on the remaining piece when the anchor extension suddenly stops (sling length runs out) ?  Let's assume a worst-case personal tether (dyneema sling or PAS) for the belayer and/or 3rd person.  And let's assume the belay device is cammed (so doesn't slip before the anchor hitch).

I'm finding it hard to convince myself that this is insignificant. But, in everything I've read about girth/clove hitch anchors, I don't remember seeing either (a) a warning about this or (b) analysis showing it's not a problem. Is it out there ?

(There is no shortage of general advice against FF2 or preferences for tethering with the rope instead of PAS, but I'm assuming an ideal anchor should avoid making things worse even in that unfortunate scenario; I think BFK does)

If ur hanging on ur clove at the belay, not much.  If you put yourself in a situation where you are exposed to FF2, and you (belayer) are not weighting your tether, this means you are probably at a large ledge, in which case ur buddy is going to end up pretty broken (and belayer will get dragged/banged up too) and who gives a frick about the maximum speed the belayer sees?

Why do u care about this?
is there a velocity that would make u feel OK about the leader taking a FF2, if so you should check yourself! Respectfully!

I would worry less about the speed of the belayer during FF2 and worry more about doing everything u can to not allow a FF2 fall to happen, and making bombproof anchors.

i just cannot understand the utility of knowing the information that you are wondering about….  

If your leader takes a FF2 onto your hanging gear belay, you're potentially going to have much bigger problems than hypothetical forces that the belayer then exerts on remaining pieces (if any) of a shock loaded gear anchor. I agree with Nathan though, the belayer is going to add pretty insignificant forces to an anchor that's already failing whether it be partially or fully.

Make it your standard practice for the leader to place a first piece (aka "jesus piece") prior to leaving the belay so that any potential fall right off the belay hits the piece first rather than the anchor. 

With 2-5 kN of resistance, the fall would continuously slow as the girth/clove slipped, energy would be absorbed, and the force experienced if the carabiner reaches the end should be less than the load that would have been felt if the fall had been stopped more abruptly without the masterpoint sliding (if a BFK were used). The clove/girth masterpoints have some weaknesses, but increased shockloading is not one of them. I’d be more concerned about decreased redundancy if one of the legs is cut, or the heat generated by the friction of the clove-X variant damaging the sling. Yann Camus has a really good video with a bunch of tests examining how these masterpoints behave when sliding.

Clove/girth testing

the one time that i belayed an FF2 i was just clove hitched into the master point, and the clove hitch was the absolute least of my problems.