Buffer Clear Rate Differs By Slot 2 Function

OK, folks, I've not experienced this before, and I can't seem to find if this is normal. I've replicated this for both the D600 and D610, but haven't checked it for other cameras yet. Camera is set up to shoot RAW images only, not RAW plus JPEG. Slot 1 has a 92Mbs Sandisk Extreme Pro card, Slot 2 has an EyeFi X2.

Scenario 1 - Slot 1 is set for RAW, Slot 2 is set for JPEG, shooting only RAW. Camera fires ~16 images before slowing down and takes at ~20 seconds for the buffer to clear.

Scenario 2 - Slot 1 is set for RAW, Slot 2 is set for Overflow, shooting only RAW. Camera fires ~22 images before slowing down and takes ~4 seconds for the buffer to clear (green light to go off).

Is this normal?!

I don't ever remember running into this. Nothing is being written to Slot 2, so why is the clear rate taking so long? Does the camera do the JPEG processing anyway, even if it's not going to write it? It seems ridiculous.

I'm not complaining as much as I am trying to understand what's going on. I looked for something on the web about this but came up empty.

Just a guess, but the Jpeg conversion makes sense.

I did some digging in the manual and it appears that when something other than Overflow is chosen the second slot is dedicated to that purpose, so perhaps it also divides the processing and buffer emptying resources based on the function and not based on what the camera is actually slated to produce? I can see that it would do that with Backup selected for Slot 2, but not this - though I suspect it was a hell of a lot easier to implement.

Tried one more thing, and if you remove the second card then it performs the same. So having the card in Slot 2 is what triggers the function, or lack of it, and not just the setting.

Interesting. I generally keep it on overflow, so optimal performance is welcome in my book.

Me too. I rarely use slot two for JPEG, but obviously had thought to do so in this case, and I think it's a shame that Nikon couldn't somehow differentiate whether or not JPEG's are actually being shot when in this mode, because it's a PITA to have to change 2 settings instead of just 1, and the performance difference is considerable.