A lot of the games being ported/reconstructed were made at a time where consumer hardware was up to or even more than an order of magnitude worse than what most people have access to today.

So that probably helps contribute to why they still run smoothly even in WebAssembly/WebGPU. Even if the overhead was like 5x worse (pretty sure its nowhere near that, closer to maybe 1.5-2x in certain cases), it'd probably still run pretty smooth.