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I frequently run the Disk Cleanup tool and let it compress old files. However, one of the directories it compressed, full of small icon images, is terribly slow. Is this a result of the compression or something else? (Note: No other directories are slow at this point, but I also haven't come across any other directories that were compressed this much by XP)

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Compression usually has an impact on the CPU. If you compress a lot of small files, it's even more taxing. It's normal behavior, in my opinion.

For instance, if you copy 1024 100kB files, it's going to take a whole lot longer than just copying one 100MB file. There's an overhead for each file; the same is true when compressing.

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There are over 1000 icons in this directory. Are you saying compression hurts browsing performance? Can I undo this? – th3dude Sep 9 '09 at 18:58
It has to have some impact; I usually hate dealing with small files. It tends to drag everything to a halt, even in normal circunstances... However, maybe there's more to this, I don't know. – alex Sep 9 '09 at 19:01

You mean "icon images" as in .ico files? If so, the contents of every file would be read in order for Windows Explorer to display the icon. This is likely to be slow, even if compression is not used.

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Compression affects the system in the following ways:

Increases CPU usage. When the system volume is compressed, Windows XP compresses and decompresses all data read and written respectively to the storage device. Compression and decompression require that the CPU process the data. This increases the CPU usage by 1 to 5 percent.

but there is more to be taken into consideration:

Increases size of write requests to the storage device. When you compress a volume, the size of write requests increases because the amount of data written to and read from the volume increases. This is caused by the compression, decompression, and extraction of data written to and read from the storage device.

Source (PDF)

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In some cases, compressing files on a system can speed things up. This is usually the case when a system is heavily I/O bound, and has CPU cycles to spare. By compressing files on disk, the amount of data that needs to be copied off the disk is reduced. This means that less time is spent waiting on the hard disk.

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I have never seen this in practice, but it certainly is theoretically possible, and with modern computers having four or more cores, it's increasingly likely. – ChrisInEdmonton Jan 27 '10 at 16:14
up vote -1 down vote accepted

It seems as though disabling the Windows Image Acquisition service helped greatly. Not sure why it affects performance on directories with a large number of images. This seems to work well, since my machine doesn't need this service.

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