It's definitely going to be significantly slower as a result of all the contextual lookups.

To get an idea, I repeatedly duplicated the CSS in the "tiny sandbox" until there were nearly 20,000 lines there, and then profiled the operation of changing the font-size by 1px in Firefox. Reflowing the textbox took about 1.6 seconds.

With `font-feature-settings: "calt" off` applied, so that the contextual lookups aren't being run, a font-size change for that same textbox content reflows in about 100 milliseconds.

(I don't have actual timings for the same example in Chrome, but subjectively, with "calt" disabled, the resize is near-instant, whereas with it enabled, it's quite laggy, similar to Firefox.)