Question: I am definitely more than thrilled with the Scala type inference engine, but in a real world environment:

1. How much of a performance draw back is it?
2. When is the type inferred, at compile time or runtime?

Answer: 
> 
> How much of a performance draw back is it?
> 
> 
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The run time performance is generally said to be comparable with Java. Compilation times are typically longer compared to Java [due to the complexity of the compiler](https://stackoverflow.com/a/3490538/390708).

> 
> When is the type inferred, at compile time or runtime?
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> 
> 

At compile time.

In this [video](http://www.youtube.com/watch?feature=player_detailpage&v=hiurd7KaSEI#t=546s) from about 8:50 to 12:50 Martin touches on usage in "real world" environments and performance. He notes there are many companies using Scala from small startups to large enterprises.
The question is where the problem with compile times in Scala being longer than in Java come from. Hard for me to say as I'm no compiler construction expert. What I can say is that type inference is obviously not costly in Kotlin and also not in Groovy2.0. What slows down Scala must be something else such as search for implicits.