Unless you're optimising for size, SMC'ing little pieces is definitely going to be slower. But e.g. in loops that do a few hundred iterations, saving a cycle or two by getting rid of some opaque predicates quickly adds up and the cache/pipeline-flushing overhead (few hundred cycles at most) becomes insignificant.

After all, as the parent comment says, a JIT is basically SMC'ing huge blocks of code, and it's definitely faster than the equivalent alternative of a bytecode interpreter.