PNaCl bitcode is (mostly) statically-typed and has a C-like memory model, as opposed to dynamically-typed JavaScript. Dynamic typing has a run-time performance cost, and so does garbage collection.
It's kind-of typed. They kept the basic integer and floating-point types from LLVM (i8, i32, i64, double) and pointers, but removed structures and arrays. It's "more typed" than JavaScript, but less than C++. This still means that numeric code (working with integers and floats) is going to be at least an order of magnitude faster than on JS.
