I'd agree that Swing feels heavy... but Electron feels an order of magnitude heavier.
Though, when done right, it feels fairly close to native. I earn my pay working with IDEA and if you turn off the plugins you don't need its actually quite snappy where you would reasonably expect it to be. I would bet it takes far less optimization to get there than an equivalent Electron application
Is swing poorly optimized, is it due to being in Java?
I'm not a UI dev and have not thought about it much.