Well, not every app is a game and needs things like superfast property access at all times. A typical app needs to be responsive, but "decently fast" property a

Well, not every app is a game and needs things like superfast property access at all times. A typical app needs to be responsive, but "decently fast" property access can fall well within what's needed for "responsive", plus you get all these new guarantees about unmodified state. The lower numbers in the first benchmark are still fairly fast, it's just that cached property access is so much faster (the test itself is also approaching about as microbenchmark as you can get, so we shouldn't generalize with the results too much). Really, Object.freeze, seal, etc are not widely used yet, and so aren't found much in the popular benchmarks, and so haven't received much attention from the Javascript engine writers. That will change if they start to become more popular. My understanding is that v8, for instance, just doesn't touch those after the initial code generation, so you don't get the optimizing compiler at all right now. That will change very quickly if the next version of the Kraken suite has a bunch of tests with sealed and frozen objects.