Throttle playback progress publishers to reduce UI lag

BookPlayer/Library/ItemList/Views/ItemProgressView.swift

@@ -10,6 +10,13 @@ import BookPlayerKit
 import SwiftUI
 
 struct ItemProgressView: View {
+  /// Throttle window applied to per-row progress publishers. Each visible row in a long
+  /// library list subscribes to three high-frequency progress streams; without throttling
+  /// SwiftUI re-diffs the circular indicators on every audio tick, which lags VoiceOver
+  /// and burns CPU. One second matches the existing `.folderProgressUpdated` cadence and
+  /// keeps the list feeling live.
+  private static let progressUpdateThrottleSeconds = 1
+
   let item: SimpleLibraryItem
   let isHighlighted: Bool
 
@@ -51,12 +58,14 @@ struct ItemProgressView: View {
     .onReceive(
       playerManager.currentProgressPublisher()
         .filter { $0.0 == item.relativePath }
+        .throttle(for: .seconds(Self.progressUpdateThrottleSeconds), scheduler: DispatchQueue.main, latest: true)
     ) { (_, progress) in
       self.progress = progress
     }
     .onReceive(
       libraryService.immediateProgressUpdatePublisher
         .filter { item.relativePath == $0["relativePath"] as? String }
+        .throttle(for: .seconds(Self.progressUpdateThrottleSeconds), scheduler: DispatchQueue.main, latest: true)
     ) { params in
       if let percentCompleted = params["percentCompleted"] as? Double {
         self.progress = percentCompleted / 100
@@ -67,7 +76,7 @@ struct ItemProgressView: View {
     }
     .onReceive(
       NotificationCenter.default.publisher(for: .folderProgressUpdated)
-        .throttle(for: .seconds(1), scheduler: DispatchQueue.main, latest: true)
+        .throttle(for: .seconds(Self.progressUpdateThrottleSeconds), scheduler: DispatchQueue.main, latest: true)
         .filter { notification in
           guard
             item.type != .book,

BookPlayer/Player/ViewModels/PlayerViewModel.swift

@@ -21,6 +21,13 @@ enum PlayerSheetStyle: String, Identifiable {
 
 @MainActor
 final class PlayerViewModel: ObservableObject {
+  /// Throttle window applied to high-frequency playback-progress notifications
+  /// (`.bookPlaying`, `.listeningProgressChanged`) before they trigger a recompute of the
+  /// progress UI. Tuned to a roughly 2 Hz refresh — fast enough for the time/progress
+  /// readout to feel live, slow enough to keep the main thread out of the audio engine's
+  /// per-tick firehose.
+  private static let progressNotificationThrottleMs = 500
+
   @Published var progressData = ProgressData()
   @Published var isPlaying = false
   @Published var playbackSpeed: Float = 1.0
@@ -223,15 +230,15 @@ final class PlayerViewModel: ObservableObject {
 
     self.playingProgressSubscriber?.cancel()
     self.playingProgressSubscriber = NotificationCenter.default.publisher(for: .bookPlaying)
-      .receive(on: DispatchQueue.main)
+      .throttle(for: .milliseconds(Self.progressNotificationThrottleMs), scheduler: DispatchQueue.main, latest: true)
       .sink { [weak self] _ in
         guard let self = self else { return }
         self.recalculateProgress()
       }
 
     self.listeningProgressSubscriber?.cancel()
     self.listeningProgressSubscriber = NotificationCenter.default.publisher(for: .listeningProgressChanged)
-      .receive(on: DispatchQueue.main)
+      .throttle(for: .milliseconds(Self.progressNotificationThrottleMs), scheduler: DispatchQueue.main, latest: true)
       .sink { [weak self] _ in
         guard let self = self else { return }
         self.recalculateProgress()