he/him 
A common misconception is that type annotations makes your program run faster. It’s false. It makes your compiler run faster. It provides *less* information than inference so, if a compiler relies on it, it probably makes it run *slower* than it could be with inference.
Why can’t the type system use annotations for type checking but inference for optimisation?
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It can but the annotations isn’t what gave you the speed in that case. I’m only addressing the misconception that adding more of them will make your code faster. Guards could in theory help locally, but obviously worse overall than knowing them statically.
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