Writing for readers who cannot run JavaScript
Most crawlers that read for an answer engine do not execute scripts. That is a content constraint, not a performance one.
The audience I keep forgetting is the answer engine that reads a page once, without a browser, and never comes back to check whether the content arrived late.
This is not a speed argument. A page that renders in eighty milliseconds after a fetch still renders in zero characters for a reader that never fetches. The rule that follows is blunt: no content may require a script to appear.
What it costs
Less than you would think, and in exactly one place. Interactivity that decorates content is free to ship, because losing it loses nothing a reader needed. Interactivity that carries content is the thing to refuse.