I am using a ModuleScript in ReplicatedStorage to organize my LocalScripts but I have noticed that you can’t use local functions in ModuleScripts and I have heard that global functions are slower than local functions. So by using global functions in a ModuleScript to call in a LocalScript, would this slow my game speed down?
No worries! Go ahead and use ModuleScripts wherever it makes sense for organizing your code. There may be a very very slight difference in access time for variables/functions in a module script from some requiring script or local script (or between a local variable and global variable in the same script), but this difference in nearly negligible and would never be perceptible from the perspective of a human player. Best practice is typically to organize your code in a way that makes sense to read as a human 
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