Bump up the optimisation level of JS

[hrj]

In my quick tests, setting the optimisation level to 9 works fine (as in no errors with a few JS tests).

However, it makes most sense to do this after other bottlenecks are fixed (since the JS optimisation will add some latency, and the benefit will be relatively marginal).

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Update: This will depend on handling of large scripts in compiled mode in rhino