I think it's a fascinating tradeoff. A typical approach with JIT-compilation is to write relatively inefficient code and hope that the JIT compiler picks up on it and optimizes it for you. Their approach seems to instead be to say "if you generate particular sequences of instructions, we'll detect those and instead execute a more efficient version, every time".

IMO this is a really cool idea because the performance optimizations are quite a bit less brittle than the ones you get with a JIT-compiled virtual machine.