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On my two machines (with SSDs) I regularly find that copying lots of small files is severely slowed down by Windows Defender. It utilizes one core to the max and when disabling it, the copy operation gets a lot faster.

I don't know about other antivirus software though.

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I can mirror this experience too. Properly working Intel SSD fwiw. Mine impacts not only file copys but also all small file I/O like listing directories (explorer would hang for 1 second entering a new directory) and the like.

I've had a couple users complain that my program started running really slowly while they were using Microsoft Security Essentials. It turns out that if a file contains a "suspicious" pattern of bytes (which in these two cases were two different user-drawn bitmaps), SE will do some time consuming heuristics every time you open the file. So opening, reading, closing a file 10 times in a row is really bad.

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