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I instrumented the code and using conditionals resulted in 2x the generation time per loop. These loops are executed only once so the VM doesn't have a chance to optimise them.

I've done similar in the past and can concur that it does have large performance benefits, at least in Chrome where I tested it.

Testing beats imagination.

Stating the obvious, but I'm pretty sure that there are no longer any purely interpreting JS engines around, it's all JIT or AOT compiled, so optimizing for a compilation scenario definitely makes sense.