It runs quite well on macOS Sonoma, and when macOS Sequoia came out, I started testing it with the new Xcode 16 beta. Unfortunately, there seems to have been some underlying changes to SwiftUI performance -- which are for the better -- but they are causing my app to lag fiercely.

I've run almost 50 different Instruments traces looking for issues between Sonoma and Sequoia, but nothing stands out to me. There's no unexplained spikes in GPU use, no unusual CPU behavior, and no noticeable flame graph changes that I could discern.

I've filed two separate bugs on Feedback Assistant (first FB14025421, then FB14562780), but neither bug has received any response from Apple engineers. I've posted on the Swift forums, but my post was (in retrospect, rightfully) marked private because it involves an Apple-owned framework [1]. I've posted on the Apple Developer forums, but received no response there [2]. I even signed up for office hours on the Apple developer portal, but my appointment was declined by Apple.

I included all of the traces I created, videos demonstrating the issue on Sequoia and the expected behavior on Sonoma, and a reproducer project in both of the bugs I've filed. I'm honestly at a loss, because I can't find any documentation or mention of changes that could be causing these issues.

So, I'm making this post as a "hail mary". If you're an Apple engineer that works with the SwiftUI or Mac Catalyst teams, or are experiencing a similar performance degradation with your app, please reach out. I'm grateful for any form of assistance that anyone can provide. Thank you in advance!

[0] https://apps.apple.com/us/app/sort-symphony/id6477886990

[1] Notably, one of the moderators said that it was an excellent question about a genuine concern, but that I should have posted on the Apple Developer forums since it was a private framework owned by Apple.

[2] https://developer.apple.com/forums/thread/758904