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And that's for each page that uses it I assume, so If you have 10 pages using Arc, that's not going to be imperceptible.

Not quite. =] Arc coordinates and synchronizes across tabs (via an iframe). So Arc behaves identically whether you have one open tab with Arc or 100.

Aren't tabs supposed to be sandboxed?

Sounds like the iframe is used to glue sandboxes together

Origin should be sandboxed rather than tabs, isn't it?

Nicely done.