Question Horrible lag on a new-ish PC with good specs

[lost-in-space]

My 3 year old PC feels like a toaster. It worked perfectly for 1 year, and suddenly started feeling like this.
If you've got any idea what this might be, or which component, please help me out.

Specs:
Intel Core i5 6402P @ 2.80GHz, Skylake
4096MB ATI Radeon RX 470 Graphics 4GB
RAM 8.00GB Single-Channel Unknown @ 1064MHz
MSI B150M BAZOOKA PLUS
SSD: 111GB Western Digital (for Windows only)
HDD: 931GB TOSHIBA HDWD110 (the only thing that stayed from the old PC, this hdd is 6+ years old)
Windows 10 Pro 64-bit

Examples of issues:

• game: r6 siege: cpu stays at 100%, game is unplayable if not on lowest resolution, but even then it still lags a lot. my specs should be great for the game though: https://www.game-debate.com/games/index.php?g_id=20387&game=Rainbow Six: Siege&p_make=Intel&p_deriv=Core+i5-6402P+2.8GHz&gc_make=ATI&gc_deriv=Radeon+RX+470+Sapphire+Nitro++4GB&ram=8&screenRes_width_FPS=1920&screenRes_height_FPS=1080&checkSubmit=#systemRequirements
• game: apex legends: after a big update, it lags the first 2-3 games (otherwise works fine)
• game: overwatch: afetr a big update, the characters won't load 1-2 minutes (otherwise works fine)
• firefox: takes 2 mins to fully load, until then, hdd usage jumps to 100%
• takes up to 2 seconds to open start
• pc will often randomly lag for a second or so

Usage - all windows closed:

• cpu: 2-5%
• ram: 31%
• ssd/hdd: 0
• gpu: 1%

Usage - only firefox running, but not actively used:

• cpu: 7-23%
• ram: 60%
• ssd/hdd: 7%
• gpu: 1%

What I've tried:

• installed windows on ssd - this only sped up the booting process, lag is still there
• reinstalled windows: did this few days ago, no issues were solved
• wiped both ssd and hdd and reinstalled everything (windows, drivers, all games), no issues solved

 

[Lutfij]

Make and model of your PSU and it's age? Which version of Windows 10 are you working with? We're currently on 20H2.

Welcome to the forums, newcomer!

 

[lost-in-space]

Make and model of your PSU and it's age? Which version of Windows 10 are you working with? We're currently on 20H2.

Welcome to the forums, newcomer!

Thanks!
PSU is Fortron Hyper 600W, I got a new one 3 years ago.

Not sure what 20H2 is, but this is the version I have:
Win10 Version 10.0.18363 Build 18363