AFAICT, the steady-state Liquid Glass effects (think: the address bar in Safari, staying static itself while you scroll the page under it) cost nothing. That's what I meant above.

Animating the Liquid Glass widgets (i.e. changing their position or shape), on the other hand, does seem to cost a lot / produce lag.

I get the impression that this is down to the UI toolkit not being optimized for whatever Liquid Glass is doing in terms of recalculating constraints during animations. (When the GPU overruns its time budget while computing shaders for the compositor, the visual effect is of [double-buffered] texture buffers dropping/repeating frames, not slowdown. Actual "lag" in a GPU-composited UI is either from CPU work, or from one-shot CUDA-type GPU "prerender jobs".)

I get the sense that Apple rushed out some shitty code that has some of these components re-evaluating a bunch of their placement and sizing constraints on every non-static animation frame (rather than just giving the Liquid Glass shaders the ability to do declarative tweens.)

Or maybe the shaders already do declarative tweens, but Apple are doing tons of redundant on-CPU per-frame recalculations, to re-do any constraint-based layout for everything around the component during the animation, that might be impacted by the component's current tweened state. I dunno.

Either way, it's definitely silly, and could be re-engineered to work a lot better.

But it's also not really "Liquid Glass's fault" (i.e. something inherent to the visual design); it's just (AFAICT) bad implementation engineering, rushed to give Apple something to talk about besides its failure to launch Apple Intelligence.

It's a very complicated system. Performance issues are bad but they're not necessarily caused by how the UI looks.

You can report an issue by typing applefeedback:// into Safari if you want.

And even if they reserved a core, that would be a waste of a perfectly good core