Linux Bessy 4.15.0-43-generic #46-Ubuntu SMP Thu Dec 6 14:45:28 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Ubuntu-mate 18.04 on a Dell Latitude 3580.
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I didn't put this in the hardware part because I'm 100% sure it isn't the hardware.
So I was having issues with processes using too much CPU. It's an intermittent problem. I installed the cpu frequency panel applet a few days ago and just today the laptop slowed down again drastically. The CPU is throttled between 400 and 750 Mhz. It will not go above that speed and I've tried a lot of suggestions online. When I simply set the governor to performance I get no improvement.
I can't find any info online that's really useful. I need to be able to use tlp and whatnot to extend battery life so I can't delete that or just set the CPU to performance and leave it there. Setting it to performance isn't helpful anyway.
The thermals are all fine. I tried an MX-Linux liveUSB I had laying around and the performance was fine.
I'm pretty stumped. If you need any other info please let me know
UPDATE & EDIT
I found out the throttling only happens when the charger is plugged in. When it's unplugged the frequency scaling goes back to normal