MacOS – 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 & 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) <not mounted>  [EFI]: 210 MB 
        (disk0s2) <not mounted>  [CoreStorage Container]: 999.35 GB
        Recovery HD (disk0s3 - Journaled HFS+) <not mounted>  [Recovery]: 650 MB 

    ST1000LM024 HN-M101MBB disk1: (1 TB) (Rotational)
    [Show SMART report]
        EFI (disk1s1 - MS-DOS FAT32) <not mounted>  [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