Only one hard drive?
That may be why.
@peru bob thx I'll try an external disk, see how that goes.
But I would imagine the mbp build in ssd with 550 Mb/s throughput (according to Black magic tool) would be sufficient.
You might want to try a workflow that works well for me on a PC laptop that is sort of similar, i7-4700HQ 2.4 GHz, 24 GB RAM, GTX 765M.
I have found that by using one of the new extremely fast Samsung portable SSD's with the USB3 interface it is very practical to edit from it as a second SSD Very few USB 3 devices have the performance to do this but the Samsung T1 and it's next generation now the T3 are phenomenal. Here are the results of the CrystalDiskMark on my 500GB drive.
I am smoothly editing 1080 AVCHD and 4K 100 Mbit/second XAVC S from these devices. I do not have access to XAVC that you refer to but I think that your A7ii actually is XAVC S.