The Magento Compiler is an outdated solution to reduce filesystem impact caused by PHP class loading. While still included in current Magento releases it is removed in Magento 2. On current systems there are even cases where the Compiler causes harm to general performance of Magento.
This Tag is related to all questions and problems caused by Usage of the Magento Compiler.
The Magento Compiler when activated puts all used php Class Files together and groups them, so the files loaded in the end are much less.
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