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copied!<p>Ultimately what it comes down to is that <code>Expression<T></code> is not a pre compiled delegate. It's only an expression tree. Calling Compile on a <code>LambdaExpression</code> (which is what <code>Expression<T></code> actually is) generates IL code at runtime and creates something akin to a <code>DynamicMethod</code> for it.</p> <p>If you just use a <code>Func<T></code> in code, it pre compiles it just like any other delegate reference.</p> <p>So there are 2 sources of slowness here:</p> <ol> <li><p>The initial compilation time to compile <code>Expression<T></code> into a delegate. This is huge. If you're doing this for every invocation - definitely don't (but this isn't the case, since you're using your Stopwatch after you call compile.</p></li> <li><p>It's a <code>DynamicMethod</code> basically after you call Compile. <code>DynamicMethod</code>s (even strongly typed delegates for ones) ARE in fact slower to execute than direct calls. <code>Func<T></code>s resolved at compile time are direct calls. There's performance comparisons out there between dynamically emitted IL and compile time emitted IL. Random URL: <a href="http://www.codeproject.com/KB/cs/dynamicmethoddelegates.aspx?msg=1160046" rel="nofollow">http://www.codeproject.com/KB/cs/dynamicmethoddelegates.aspx?msg=1160046</a></p></li> </ol> <p>...Also, in your stopwatch test for the <code>Expression<T></code>, you should start your timer when i = 1, not 0... I believe your compiled Lambda will not be JIT compiled until the first invocation, so there will be a performance hit for that first call.</p>

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