While running a scan, it will use most of your hardware resources. Msmpeng.exe is windows defender program, if you see this while running a scan, then it is completely okay, but randomly it starts slowing down your system you can try restricting Windows Defender from scanning its directory or use alternative antivirus to check if you are hit by any malware. see this article on msmpeng.exe
I can see this process with huge cpu usage and my system is slowing down like hell.

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