Laggy write/read performance w/ Samsung 850 evo 1TB on mid-2012 Macbook Pro 10.12.6 Sierra Here's a weird one: after a Samsung 840 pro 512gb has defaulted this week (applications didn't open, files couldn't be accessed any longer), I bought a Samsung evo 1TB SSD and installed it in my Macbook Pro. I restored from a time machine backup which worked fine. 

The issue now is this: Using Blackmagic, I see appropriate write speeds, but only about half of the time (around 500MB/s write and read). The other half it's hovering around 200-300, and performance in general is worse than with the older Samsung 840 pro I had installed before. 

When opening Finder windows and performing simple tasks I notice a lag that occasionally occurs, making the system as a whole sluggish and slow. Here's a youtube video showing the issue when using Blackmagic (any visible lag is from my system, not the video recording): 

https://youtu.be/AX9bZSZvIRc

Has anybody experienced similar issues or does anybody have an idea what could be the root cause? 

Some more info: 


Trim is enabled
Disk utility does not show any issues
I have run both Onyx and TechTool Pro 9 to resolve any common issues such as partition map verification, ownership rights etc.
the only thing I have noticed is that the inspector window shows that my user account does not have write access to the ssd (see image below), when I try to change that it says "The operation cannot be completed because you don't have the necessary permission", which is weird since I am the admin. Could it be related to this, and how would I got about changing that? 




I am thankful for any hints, what other components could be the bottleneck here? Maybe the connection cable of the SSD to the motherboard? A faulty SSD controller chip? 

Here's the Ethrecheck file: 

EtreCheck version: 3.4.2 (436)
Report generated 2017-08-18 18:53:19
Download EtreCheck from https://etrecheck.com
Runtime: 4:00
Performance: Good


Hardware Information: ⓘ
    MacBook Pro (13-inch, Mid 2012) 
    [Technical Specifications] - [User Guide] - [Warranty &amp; Service]
    MacBook Pro - model: MacBookPro9,2
    1 2,9 GHz Intel Core i7 (i7-3520M) CPU: 2-core
    16 GB RAM Upgradeable - [Instructions]
        BANK 0/DIMM0
            8 GB DDR3 1600 MHz ok
        BANK 1/DIMM0
            8 GB DDR3 1600 MHz ok
    Bluetooth: Good - Handoff/Airdrop2 supported
    Wireless:  en1: 802.11 a/b/g/n
    Battery: Health = Normal - Cycle count = 34

Video Information: ⓘ
    Intel HD Graphics 4000 - VRAM: 1536 MB
        Color LCD 2560 x 1440
        DELL U2713HM 2560 x 1440 @ 59 Hz

Disk Information: ⓘ
    Samsung SSD 850 EVO 1TB disk0: (1 TB) (Solid State - TRIM: Yes)
    [Show SMART report]
        EFI (disk0s1 - MS-DOS FAT32) &lt;not mounted&gt;  [EFI]: 210 MB 
        (disk0s2) &lt;not mounted&gt;  [CoreStorage Container]: 999.35 GB
        Recovery HD (disk0s3 - Journaled HFS+) &lt;not mounted&gt;  [Recovery]: 650 MB 

    ST1000LM024 HN-M101MBB disk1: (1 TB) (Rotational)
    [Show SMART report]
        EFI (disk1s1 - MS-DOS FAT32) &lt;not mounted&gt;  [EFI]: 210 MB 
        HDD (disk1s2 - Journaled HFS+) /Volumes/HDD : 500.10 GB (76.47 GB free)
        HDD2 (disk1s3 - Journaled HFS+) /Volumes/HDD2 : 499.62 GB (4.67 GB free)


Virtual disks: ⓘ
    SSD (disk2 - Journaled HFS+) /  [Startup]: 998.98 GB (463.40 GB free)
        Physical disk: disk0s2 999.35 GB Online

System Software: ⓘ
    macOS Sierra  10.12.6 (16G29) - Time since boot: about one hour


Top Processes by Memory: ⓘ
    1.35 GB     kernel_task
    458 MB      Google Chrome Helper
    392 MB      Google Chrome Helper
    389 MB      Google Chrome Helper
    375 MB      Google Chrome Helper

Top Processes by Network Use: ⓘ
    Input       Output      Process name
    169 MB      43 MB       openvpn
    76 KB       36 KB       mDNSResponder
    28 KB       52 KB       usbmuxd
    9 KB        5 KB        netbiosd
    96 B        96 B        ntpd

Top Processes by Energy Use: ⓘ
     16.28  Google Chrome Helper
     15.90  WindowServer
     12.36  Google Chrome
      7.86  Google Chrome Helper
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.