When I worked in a restaurant kitchen, we used to soak rags with water and freeze them in the walk in freezer, then once it’s nice and frozen we’d wear the rag around our necks.
There’s large blood vessels in the neck feeding your brain, so if you’re able to cool down the blood there, it’ll spread to the whole body surprisingly fast.
I actually managed to get cold in hot humid july summer in the kitchen with that method.
egui is cool but it’s an immediate UI kind of thing, that’s usually used in video games as you’re constantly re-rendering the whole thing anyway.
A more fitting UI library would be Iced, which is also what System76 is building their COSMIC desktop environment on.
It does claim to support the web too, although not via HTML which brings some accessibility concerns. I’d expect performance to be very good otherwise.