Neither. Top level generators in Observable are processed via requestAnimationFrame. Safari limits the frame rate (as well as timeouts) until the first user interaction. If you want to utilize the full frame rate you’ll have to make your users start the animation.
You can find more details here:
- safari framerate cap in rendered notebooks pre-interaction
- Runtime stops executing when in background · Issue #458 · observablehq/feedback · GitHub
For animations I would generally recommend to design them in a way that lets you produce a frame for any given point in time. That way you can define the desired duration, and then calculate the current animation offset since the animation start on every yield.