clouds95 wrote:
I just bought a used Early 2015 MacBook Pro from eBay and the system came fresh with macOS Monterey, I noticed it was running very slow so I reinstalled the OS but it didn't fix the issue.
Resizing windows aren't smooth and jitter a lot, opening new tabs takes 3 seconds to load, applications seem to take a while to process, loading websites freeze a little bit and take a little bit to load, if I open the WiFi networks tab and start moving my cursor up and down fast, you can see the networks being highlighted slowly and don't match where my cursor is at
The problems continues even in Safe Mode and Recovery Mode.
Here's my EtreCheck Report.
<EtreCheck Report.log>
You can try disabling or removing those login items on startup and compare your results...

I see no obvious reasons.
You can Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support This will tell you if it a universal issue or isolated to your user/admin account.
re: < disk0 - APPLE SSD SM0256G 251.00 GB (Solid State - TRIM: Yes) >
the options were: 128, 256, 512 GB or 1 TB flash storage
this all seems normal enough for your 256GB
Write speed: 1241 MB/s
Read speed: 1456 MB/s
compared to another post I see with comparison—
13" Macbook Pro w/ 128 GB SSD:
640 MB/s Write,
1400 MB/s Read
13" Macbook Pro w/ 512 GB SSD:
1350 MB/s Write,
1400 MB/s Read
In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment
Make an appointment for a "hardware issue"
Genius Bar Reservation and Apple Support Options - Apple
Outside the USA
Contact Apple for support and service - Apple Support