Laggy write/read performance w/ Samsung 850 evo 1TB on mid-2012 Macbook Pro 10.12.6 Sierra
I found the culprit as follows:


did a hardware test by holding 'd' key on startup
did extended system test (short one passed)
test kept on crashing when testing memory
switched both 8gb RAM sticks against new ones


Now the system works again as expected.