Javascript performs unchecked memory accesses? Or are you alluding to their both being weakly typed?

Which is ultimately irrelevant here. Consider, C is rock solid safe compared to machine/assembly.

(Though, I tend to share the annoyance of javascript...)

Oh it does have pretty good performance, on par with hand-written JavaScript. But that's still 3x slower than Scala/JVM on our benchmarks, and those benchmarks are ported from the Octane benchmark suite used by V8 (so supposedly that's where V8 shines).

Everything is relative. Scala.js remains significantly slower than Scala/JVM in terms of throughput.