diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryPresenter.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryPresenter.kt
index 5f9a2ea428..bba17d7f9d 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryPresenter.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryPresenter.kt
@@ -47,7 +47,7 @@ class ReactionSummaryPresenter @Inject constructor(
         fun handleEvents(event: ReactionSummaryEvents) {
             when (event) {
                 is ReactionSummaryEvents.ShowReactionSummary -> target.value = ReactionSummaryState.Summary(
-                    reactions = event.reactions,
+                    reactions = event.reactions.toImmutableList(),
                     selectedKey = event.selectedKey,
                     selectedEventId = event.eventId
                 )
@@ -73,8 +73,8 @@ class ReactionSummaryPresenter @Inject constructor(
                             avatarUrl = member?.avatarUrl
                         )
                         sender.copy(user = user)
-                    })
-                })
+                    }.toImmutableList())
+                }.toImmutableList())
             }
         }
     }
diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryState.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryState.kt
index 1d1606e53d..1ccfdc9abf 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryState.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/reactionsummary/ReactionSummaryState.kt
@@ -18,13 +18,14 @@ package io.element.android.features.messages.impl.timeline.components.reactionsu
 
 import io.element.android.features.messages.impl.timeline.model.AggregatedReaction
 import io.element.android.libraries.matrix.api.core.EventId
+import kotlinx.collections.immutable.ImmutableList
 
 data class ReactionSummaryState(
     val target: Summary?,
     val eventSink: (ReactionSummaryEvents) -> Unit
 ) {
     data class Summary(
-        val reactions: List<AggregatedReaction>,
+        val reactions: ImmutableList<AggregatedReaction>,
         val selectedKey: String,
         val selectedEventId: EventId
     )
diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/factories/event/TimelineItemEventFactory.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/factories/event/TimelineItemEventFactory.kt
index aea1019f11..c5d303b3f1 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/factories/event/TimelineItemEventFactory.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/factories/event/TimelineItemEventFactory.kt
@@ -117,6 +117,7 @@ class TimelineItemEventFactory @Inject constructor(
                             sentTime = timeFormatter.format(date),
                         )
                     }
+                    .toImmutableList()
             )
         }
         // Sort aggregated reactions by count and then timestamp ascending, using
@@ -127,7 +128,9 @@ class TimelineItemEventFactory @Inject constructor(
                 compareByDescending<AggregatedReaction> { it.count }
                     .thenBy { it.senders[0].timestamp }
             )
-        return TimelineItemReactions(aggregatedReactions.toImmutableList())
+        return TimelineItemReactions(
+            reactions = aggregatedReactions.toImmutableList()
+        )
     }
 
     private fun MatrixTimelineItem.Event.computeReadReceiptState(
diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReaction.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReaction.kt
index 59c52ed8cf..cc6e956a1c 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReaction.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReaction.kt
@@ -18,6 +18,7 @@ package io.element.android.features.messages.impl.timeline.model
 
 import io.element.android.libraries.core.extensions.ellipsize
 import io.element.android.libraries.matrix.api.core.UserId
+import kotlinx.collections.immutable.ImmutableList
 
 /**
  * Length at which we ellipsize a reaction key for display
@@ -35,28 +36,22 @@ private const val MAX_DISPLAY_CHARS = 16
 data class AggregatedReaction(
     val currentUserId: UserId,
     val key: String,
-    val senders: List<AggregatedReactionSender>
+    val senders: ImmutableList<AggregatedReactionSender>
 ) {
     /**
      * The key to be displayed on screen.
      *
      * See [MAX_DISPLAY_CHARS].
      */
-    val displayKey: String by lazy {
-        key.ellipsize(MAX_DISPLAY_CHARS)
-    }
+    val displayKey: String = key.ellipsize(MAX_DISPLAY_CHARS)
 
     /**
      * The number of users who reacted with this key.
      */
-    val count: Int by lazy {
-        senders.count()
-    }
+    val count: Int = senders.count()
 
     /**
      * True if the reaction has (also) been sent by the current user.
      */
-    val isHighlighted: Boolean by lazy {
-        senders.any { it.senderId.value == currentUserId.value }
-    }
+    val isHighlighted: Boolean = senders.any { it.senderId == currentUserId }
 }
diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionProvider.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionProvider.kt
index dcd6bb105c..087de1753a 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionProvider.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionProvider.kt
@@ -18,6 +18,7 @@ package io.element.android.features.messages.impl.timeline.model
 
 import androidx.compose.ui.tooling.preview.PreviewParameterProvider
 import io.element.android.libraries.matrix.api.core.UserId
+import kotlinx.collections.immutable.toImmutableList
 import java.text.DateFormat
 import java.util.Date
 
@@ -53,6 +54,6 @@ fun anAggregatedReaction(
     return AggregatedReaction(
         currentUserId = userId,
         key = key,
-        senders = senders
+        senders = senders.toImmutableList()
     )
 }
diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionSender.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionSender.kt
index 276ee0b266..987474bf04 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionSender.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/model/AggregatedReactionSender.kt
@@ -16,10 +16,12 @@
 
 package io.element.android.features.messages.impl.timeline.model
 
+import androidx.compose.runtime.Immutable
 import io.element.android.libraries.matrix.api.core.UserId
 import io.element.android.libraries.matrix.api.user.MatrixUser
 import java.util.Date
 
+@Immutable
 data class AggregatedReactionSender(
     val senderId: UserId,
     val timestamp: Date,