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?
> 
> 
> 

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.
This is mentioned in this answer by Martin Odersky : [Why does IntelliJ IDEA compile Scala so slowly?](https://stackoverflow.com/questions/3606591/why-does-intellij-idea-compile-scala-so-slowly) "Type inference is costly, in particular if it involves implicit search."