CPU went bad after system changes.

[ThetaSigma was taken]

(First time using a forum. Sorry for any bad practices or idk, lack of any standard forum etiquete)
I'll try to be efficient here, so you can give me all the initial tests required, as I'm already expecting a lot of those.

I'm running a Dell Inspiron 5558 (i7 5500u with NVIDIA 920m). It was working fine, or as much as a laptop can with all it's thermal limitations. I had totally forgot about my old laptop after getting this one some time ago and then I realized I had a SSD installed there. The laptop only has one drive slot, so I did a thing that is pretty common, at least here XD. I removed my dvd from the slot and used the extra sata entrance to plug in the SSD and installed my windows 10. Also, all my original 8GB of RAM were in only one of the slots, so I removed one 4GB RAM stick from the old laptop and put it there as they were both DDR3 and same frequency.

Here starts the problem. Almost every system request uses too much CPU. I'm not a heavy gamer or anything like that. My specs wouldn't even allow me that. The thing is that even when I'm just playing League in one monitor with chrome open on the secondary, my system hits 95-99% some times and things starts to stutter a bit. And unless my old laptop (i5 with only intel graphics) is some magical being, there's something wrong going on because it could do all this and more just fine with the same SSD and less RAM.

And here is all the information I have that might help from the start:

- New installed RAM passed Dell's BIOS test, both could work together at their original 1600mhz.
- System usage reports disagree. Task manager says 80% of CPU is being used, individual programs percentages don't add up to this but Xbox game bar says it's at 100%.
- Yes, my power options are set to use 100% of CPU. I even tried in between levels just to be sure, nothing changed besides throttling if set lower, as expected.- Yes, my drivers and Windows are up to date. I also tried downgrading graphics drivers or rolling back windows to previous versions, not a change.
- Used throttlestop to see if I could squeeze any temps, but can't go too far on a average laptop and it isn't overheating like crazy anyway.
- Before all of this, on the previous windows install with a hard drive only, I used to have a random BSOD saying "driver power state failure" After the new install, it stopped for almost 2 weeks when I just had one while writing this post.
- Also, regarding Dell's BIOS, it has a hardware test that found "errors" but doesn't say what it is, just give me an OK button and theres no way to read a report about it on BIOS. If you want that I need instructions on how to get it.

TLDR I moved my SSD to a better laptop, added a RAM stick, clean install of windows and now my CPU screams at any load presented to it like it's an LHC simulation.

I'll need basic instructions on how to get reports/logs from things since I always solved basic problems myself and never needed to send those to anyone. Thanks in advance.