To my understanding today it depends on the JS engine and more importantly what JIT stage that code is in. JITs optimize const/let in ways that they can't optimize var now.
Which is to say that if someone tells you that you should in general never use const/let for "performance" they are probably wrong, but yes there probably are still edge cases and microbenchmarks that make var look faster on some engines.
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To my understanding today it depends on the JS engine and more importantly what JIT stage that code is in. JITs optimize const/let in ways that they can't optimize var now.
Which is to say that if someone tells you that you should in general never use const/let for "performance" they are probably wrong, but yes there probably are still edge cases and microbenchmarks that make var look faster on some engines.