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It ‘s difficult to say about how much does it faster but it ‘s faster. The speed will belong to many factors so that to know how much does it faster you only have way to do the benchmark before and after you do the change.
If you have a choice between APC or varnish and the compiler, use APC or varnish. You’ll get a much larger speed gain.
A lot of the time you have no choice when a much needed third party module holes your website below the waterline due to an incipient compiler incompatibility and you find you have to run without the compiler.
Do some actual AB testing and don’t take the cargo cult answers as gospel. Depending on your server setup, compilation may actually slow you down.