I'm mainly referring to how GTK is a very high level toolkit with many layers of abstractions and a number of complex functionalities built-in by default. Whereas iced in comparison is a low level library where you have to bring your own toolkit, or do it raw.

You might be surprised by how heavy some of the more established GUI toolkits are, which you don't see because the abstractions are hidden from view in dozens of event loops running asynchronously in the background on the same local thread.

Iced widgets are collectively compiled down to a single state machine that pushes messages to a streamlined singular event loop. Commands and subscriptions are spawned on background thread(s). So the API by design keeps the UI thread free to focus on the UI.

The Elm model gives you freedom to manage all of your memory in a central location efficiently. So you can easily render a complex responsive UI at 240 FPS even if you're using software rendering. How fast it renders is going to depend on how you cache and reuse your data.

You can pass data by reference to widgets so they're drawn without allocating. You can load images on a background thread and cache them in your app struct so they only need to be decoded once. You can use lazy widgets, and even watch window dimensions to render most things in a single pass. Allocations are unfortunately required when you're attaching multiple elements to a container, but that can change once Rust's allocator API is stabilized.

iced does also internally cache some things itself between frames so they don't have to be re-drawn. And it'll surely get more sophisticated in this over time. Especially once damage-tracking is fully implemented.

And if you want a higher level abstraction with platform integrations built in, libcosmic is coming around the corner as a high level platform toolkit for making COSMIC applications with iced that's tightly integrated with the COSMIC ecosystem. So you don't have to worry about how you're going to render client-side decorations, integrate with window manager protocols, implement theme support, configuration APIs, accessibility, etc.