What is MsMpEng.exe? When Task Manager isn't open my computer sometimes doesn't run as well as it would when Task Manager is open.

Dear Ath818

Thank you for posting in the Microsoft community.

MsMpEng.exe is a part of Microsoft Windows component that represents the core process of Windows Defender called Microsoft Malware Protection Engine.The main function of this process is to perform real-time anti-malware scans to protect your computer from viruses, spyware and other potentially harmful software threats.

Possible Problems and Phenomena

1. High CPU utilization:

High CPU utilization may occur because Windows Defender is performing a system scan, especially when it is working more aggressively when launching or installing new applications.

Such scanning involves extensive checking of the file system and therefore consumes a lot of processing resources.

2. CPU utilization drops when Task Manager is open:

When you open Task Manager, the program basically prioritizes the display of the most currently active processes, and certain background processes may reduce their activity to save resources.

MsMpEng.exe may reduce its scanning priority at this point to allow Task Manager to provide real-time resource usage.

Overall, MsMpEng.exe's high resource usage is usually the result of normal security maintenance. It means that Defender is performing security maintenance and background scanning to keep your computer safe.

Hope the above information can help you.

Best Wish

Shawn.Z-MSFT | Microsoft Community Support Specialist