diff --git a/flutter_readium/CHANGELOG.md b/flutter_readium/CHANGELOG.md
index 53958f9d..d2bd2bb2 100644
--- a/flutter_readium/CHANGELOG.md
+++ b/flutter_readium/CHANGELOG.md
@@ -5,6 +5,12 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased
 
+### Fixed
+
+- **Creating `ReadiumReaderWidget` could freeze the Android UI while the native reader
+  initialized.** Publication positions, which Readium's EPUB fragment constructor reads with a
+  blocking call, are now fetched and cached on the I/O dispatcher first.
+
 ## [0.4.2] - 2026-08-28
 
 ### Fixed
diff --git a/flutter_readium/android/src/main/kotlin/dk/nota/flutterreadium/ReadiumReader.kt b/flutter_readium/android/src/main/kotlin/dk/nota/flutterreadium/ReadiumReader.kt
index da8e9ebb..390006fb 100644
--- a/flutter_readium/android/src/main/kotlin/dk/nota/flutterreadium/ReadiumReader.kt
+++ b/flutter_readium/android/src/main/kotlin/dk/nota/flutterreadium/ReadiumReader.kt
@@ -54,6 +54,7 @@ import org.readium.r2.shared.publication.Locator
 import org.readium.r2.shared.publication.LocatorCollection
 import org.readium.r2.shared.publication.Publication
 import org.readium.r2.shared.publication.html.cssSelector
+import org.readium.r2.shared.publication.services.positionsByReadingOrder
 import org.readium.r2.shared.publication.services.content.DefaultContentService
 import org.readium.r2.shared.publication.services.content.content
 import org.readium.r2.shared.publication.services.content.contentServiceFactory
@@ -953,6 +954,14 @@ object ReadiumReader :
         // already disables synchronization, start in manual mode.
         narrationSyncEnabled = initialPreferences.disableSynchronization != true
 
+        // EpubNavigatorFragment 3.3.0 reads this position list with runBlocking from its
+        // constructor. For a streamed Web Publication that can be a network request, which would
+        // otherwise park Android's main thread for seconds or minutes. Populate the service cache
+        // first on the I/O dispatcher so the constructor's lookup is immediate.
+        withIOContext {
+            pub.positionsByReadingOrder()
+        }
+
         withMainContext {
             epubNavigator?.let {
                 attachEpubNavigator(fragmentManager, viewGroup)