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tweak won't do much of anything for a system without a significant amount of RAM (if you don't have more than 128 MB, don't even bother), but systems with more than 128 MB of RAM will generally find a performance boost by setting this to between 8 and 16 MB. The default is 0.5 MB, or 512 KB. This setting requires a value in bytes, so multiply the desired number of megabytes * 1024 * 1024. That's X * 1048576 (where X is the number, in megabytes). Test out several settings and keep the one which seems to work best for your system.