"Run". Yes, just ~600 times slower. At that speed it probably won't be very useful. You have ~50 msec for an operation which is perceived as instantaneous. If it takes 30 seconds (600 times as long), people will not use it. It simply isn't feasible.

This stuff is only ES3 compatible because it didn't need anything from ES5. A low-level compiler target doesn't need any of these things: http://kangax.github.com/es5-compat-table/