Replacing 1T OS SSD with 2T OS drive causes new drive to run slower

I was trying to upgrade my OS SSD on my new XPS 8960 and gave up. I cloned the Dell 1T OS SSD to a new, faster, 2T SSD using EASEUS Partition manager. I then discovered that Bitlocker is turned on causing all my disks to be locked out. I temporarily turned off Bitlocker and tried again and successfully booted the system but the fast 2T SSD that had read at 7000 MB/s before when tested on SSD slot 2 as a standard (non-OS) disk now ran at 4000 MB/s. This is versus the original Dell OS SSD that runs around 6000 MB/s (all speeds are read using SEQ1M speed test on CrystalDiskMark 8). 

I gave up. 

I'm guessing that Dell has made it difficult/impossible to upgrade the OS SSD? Do I have to buy a SSD through them?

As I said, the new M2 SSD ran fast in Slot2 as a standard disk. When cloned to an OS disk and run in Slot1, speeds dropped from 7000+ to 4000.

I tried two different brands of 2T SSD, both by reputable and highly rated hardware companies. Both had the same behavior.

Any help by the community would be appreciated.

I'm now back to the original SSD which has enough space for the foreseeable future, especially since I'll be using Slot2 for most of my work and reserving the OS SSD for programs only.

Didn't have this problem in my older Dell Inspiron. I was able to upgrade the OS SSD.

Note that I was successfully able to use M2 Slot2 and am having now problems in that area.