Also, as of a few years ago, the only performance-related reason why the JVM wasn't a popular language for desktop GUI apps was startup time.
I don't think so. Besides startup time, Swing was always slow --an over-engineered mess. For some Java people it was always "fast enough in the latest version" (like for some Linux people it was always "the year Linux wins over the Desktop"), but even the best Swing UI had perceptible lags over a bog standard native. Heck, even SWT that's half-native has huge GC related lags in Eclipse.
Swing also had the uncanny valley effect, trying to mimic native UIs. And even when they tried to bypass the issue with custom l&f like Alloy et al, they couldn't, because the uncanny value is mostly due to how the controls BEHAVE and not with their style (that's why in, say, OS X, you can use apps styled like Aqua and others styled like Metal at the same time and you don't get the "uncanny valley" effect).
If we judge Java by Eclipse, can we judge C by iTunes? :-) They both tend to become unresponsive at odd times, but it's caused by clumsy background processing, not language performance.
The Eclipse framework itself is plenty fast, and UIs based on Eclipse RCP can be quite snappy. (Except for that damned startup time.) Swing's a mess, but if you're looking for the technical limitations of a language platform, it's the best performers that are relevant, not the worst performers. Otherwise, iTunes is evidence that even C is just too slow.
I don't think so. Besides startup time, Swing was always slow --an over-engineered mess. For some Java people it was always "fast enough in the latest version" (like for some Linux people it was always "the year Linux wins over the Desktop"), but even the best Swing UI had perceptible lags over a bog standard native. Heck, even SWT that's half-native has huge GC related lags in Eclipse.
Swing also had the uncanny valley effect, trying to mimic native UIs. And even when they tried to bypass the issue with custom l&f like Alloy et al, they couldn't, because the uncanny value is mostly due to how the controls BEHAVE and not with their style (that's why in, say, OS X, you can use apps styled like Aqua and others styled like Metal at the same time and you don't get the "uncanny valley" effect).