Electron apps are memory heavy and slow at interaction because Chromium loads a ton of rendering and Web API code into memory, and interactions triggered by keyboard/mouse/touch input go through layers of non native handlers so that event listeners behave uniformly across devices. It's all part of a complex rendering loop, and whether an element has a fixed position during scroll can affect memory and performance by hundreds of times compared to those 2x js-vs-native you mention.

Oh, and the Web API has has stuff you wouldn't expect like USB, Bluetooth, MIDI, vibration, TPM etc.