I just spent hours trying to figure out and fix a Windows 10 machine that was taking 100% disk, 100% RAM and about 50% CPU with absolutely nothing running. Of course, I initially blamed the user... then, a misbehaving (hidden, background) app... then, malware / virus... the, a runaway Windows Defender... then, a hard disk / hardware failure... but ultimately discovering the culprit is - drum roll - Microsoft Windows itself! As usual!
To cut a long story short - just go ahead and disable the SysMain service. Previously this was called SuperFetch but now, Redmond feels it’s more appropriately called something senseless. What does this process do? Who knows. Does it work advertised (“Maintains and improves system performance over time”)? In many cases, it does the opposite. Do we need it? Nope.
The process runs before logon even happens, and chews through all system RAM and fully utilizes the disk to such an extent that it takes minutes (I’d say 5-10) to even get past the login. It keeps going, indefinitely, regardless of what you want to actually use your computer for!
Strangely Task Manager and other tools did not lead me to any obvious conclusion - nowhere could I find any indication that SysMain was the culprit.
Don’t blindly follow this or any other advice! I don’t validate my content and content in the links below!
Save yourself hours of grief and try this first: Open Services.msc and select SysMain > Properties. Hit Stop and under Startup Type, select Disabled. Ok. Reboot.
Search for more info on-line. I found stuff like this - What is Superfetch or SysMain ? How to disable it? How to fix it?, and this - Windows 10: SysMain Causing High Disk or High CPU Usage? Here’s How to Disable It.
Rant: You can tell I really dislike Windows 10 for many, many reasons - it is bloated, buggy, unoptimized, includes spyware a.k.a. telemetry, can’t do search, is a horrible UI mash up of Metro, Win32 forms, WPF, HTML... the list goes on. I still use Windows 7, but alas, support ended on 14 Jan 2020. Nice timing.