A7R3 hi speed mode acting strange

[craig50]

so, the camera is rated approx 10 fps and I'd say performs close to that, uncompressed raw+jpeg (raw to 1, jpeg to 2.) had a card failure a few days ago  (fortunately I was able to recover most everything). Had a marching band competition tonight, so I thought I'd set it to sequentially write uncompressed raw to both cards, in case one fails again. Other than that,, nothing is different otherwise:

CAF, Hi speed, 1/640-1/1000, ISO 1200 (lower than my usual 3200 as this is an indoor arena with much better lighting than a high wchool football field.)

dual lexar pro uhxs-ii, 256gb card, class 3 v60 so it should be plenty fast

Here's the problem. I noticed the camera would take 3-4 shots, pause then take 2-3 more. hasn't done that before. I figured it's just writing that data to the second slot which is uhs1 and not uhs ii (only slot one is uhs ii). Well, the buffer depth from what I've read shoudl be well over that and should be able to buffer 30 or so shots. I'm not shooting near that many and it's pausing, I assume it's waiting to clear the buffer as it writes to the cards. 

Just seems like it should be able to shoot more than 5-6 at a time before it locks up. I mean, I didn't when I had it set raw/jpeg. Sound right or is something else going on? I am doing controlled burst shooting, but not spray and pray.