Comment by MobiusHorizons

This is because the execution is very predictable, so the JIT in the runtime can emit optimized code with the knowledge of how the code is going to run. Embedding unpredictable code (like a javascript interpreter) typically has substantially worse performance when executing under a JIT. This is in addition to the fact that Quickjs (despite being pretty good) can't match the performance of sophisticated JIT implementations like V8 or JavaScriptCore