Apple Samsung 512 GB SSUBX SSD: Half as fast in MacBook Pro 15" 2015 than in MBPR 13" 2014? I have an Apple Samsung 256 GB SSUBX SSD which when installed in a 13" MacBook Pro Retina 2014 delivers 1,300-1,450 MB/s Read-Write, whereas when installed in newer 15" MacBook Pro Retina 2015 only provides 650-750 MB/s (both measured with Black Magic Design Disk Speed Test).

Makes somehow no sense to me... Ideas?
I should mention that I had no access to that 15" MBPR, but that that performance data was reported to me by the customer who bought this SSD from me. Looks however as if that MacBook had some serious repair damages, as the customer has now confessed. That said, I guess we can ignore my question.