Is JetBrain stack native? I don't think it is.
Is swing poorly optimized, is it due to being in Java?
I'm not a UI dev and have not thought about it much.
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
Also, Eclipse and Jet Brains software feel more similar to each other and now that you mentioned Java, that makes sense.