Window becomes unresponsive when selecting a lot of text

Since last week (I think??), selecting any larger number of text (> 20 chars should be enough) makes the window unresponsive. Scrolling, selecting other text or text input lag > 5 seconds behind.

Since there are no recent changes of Boop, I suspect an error in Mac OS / AppKit? But I'm absolutely unable to trace that, or create a Radar on that.

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