On both my Windows 8 machines, Antimalware Service Executable periodically utilizes 100% of a hard drive, and any user operations involving that drive slow to a crawl.

1.  On my Lenovo T530 (i5-ivy bridge) laptop, the service would max out my disk performance for minutes at a time on startup.  My laptop would be sluggish and unresponsive until the service quit hogging the disk.  Periodically my laptop would slow to a crawl at other times, and when checking the task manager it would always be the fault of this service utilizing 100% of my disk performance.  I eventually avoided this problem by upgrading to an SSD.

 2.  On my Xeon workstation with Adaptec hardware RAID the service actually manages to completely max out the performance on a RAID 10 array.  I was alarmed with simple copy operations stalled, and when I checked the task manager, sure enough, antimalware service executable was using 100% of the array.

Antimalware Service Executable behaves completely differently from Windows Search.  Search immediately slows to idle when user operations are underway, and does not make any noticeable impact on performance.  The Antimalware service never slows in response to user activity; it hogs the drive until it is finished with whatever it is doing.

Has anyone else observed this problem?  Is there a solution short of disabling the service entirely?

• Edited by daflory Sunday, January 27, 2013 5:48 AM

I found another solution to this problem. Under Windows Defender/Settings there is an option to exclude file types and directories.

I excluded my large data directories as well as the installed program directories (at my risk, obviously). The disk usage of the Antimalware service plummeted. My RAID queue no longer maxes out whenever I access large numbers of data files simultaneously (which I do often). You can also disable file scanning by extension.

• Marked as answer by daflory 22 hours 16 minutes ago