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"Java forces you to actually know how exactly variance works."

No it doesn't, it's optional. You can also get invariance or complete co/countervariance. But since you don't know how that works, you can't see why having complete co/counter variance for read and write operations is bad.

By the way, not only is Scala abismaly slow as compared to Java. It's abismaly slower than Java reflection/introspection libraries like SPeL. And those libraries are by themselves abismaly slower than pure Java.

"By the way, not only is Scala abismaly slow as compared to Java."

I'm sure Twitter wouldn't use it for their API backend if it was abysmally slow.

Truth is, it depends very much on what you are trying to do with Scala. It can be just as fast as Java but certain styles or patterns of coding will slow things down. See here for example:
http://stackoverflow.com/q/5901682