Lint/hints taking forever to update after code editing, autocomplete popup takes a second to show, etc. This depends on codebase naturally but in my experience for the cases where VS Code works well (mostly typescript) on the same codebase and same PC it's much more responsive than IDEA tools or Visual Studio (and I'm running this on a i9 MBP with 32GB ram so not a weak HW thing).
I type on a 60hz monitor and I'm not particularly sensitive to that kind of latency (this is where I consider VS Code good enough)
I mean sure, faster is better, but door unlock speed is probably not the problem that makes people think a car is “slow”.
If the feature becomes on par with IntelliJ, everyone would jump ship. But not today.
Also, Eclipse and Jet Brains software feel more similar to each other and now that you mentioned Java, that makes sense.
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