diff --git a/app/jobs/message/broadcast_unread_room_job.rb b/app/jobs/message/broadcast_unread_room_job.rb
new file mode 100644
index 0000000..4b0bc2c
--- /dev/null
+++ b/app/jobs/message/broadcast_unread_room_job.rb
@@ -0,0 +1,5 @@
+class Message::BroadcastUnreadRoomJob < ApplicationJob
+  def perform(message)
+    message.broadcast_unread_room
+  end
+end
diff --git a/app/models/message/broadcasts.rb b/app/models/message/broadcasts.rb
index f82e4e3..b9660c9 100644
--- a/app/models/message/broadcasts.rb
+++ b/app/models/message/broadcasts.rb
@@ -1,21 +1,27 @@
 module Message::Broadcasts
   def broadcast_create
     broadcast_append_to room, :messages, target: [ room, :messages ]
-    broadcast_unread_room
+    broadcast_unread_room_later
   end
 
   def broadcast_remove
     broadcast_remove_to room, :messages
   end
 
-  private
-    # Fanned out to the room's members rather than published on one global stream, so
-    # that the timing of activity in a room only reaches people who are in it.
-    def broadcast_unread_room
-      payload = ActiveSupport::JSON.encode(roomId: room.id)
+  # Fanned out to the room's members rather than published on one global stream, so
+  # that the timing of activity in a room only reaches people who are in it.
+  def broadcast_unread_room
+    payload = ActiveSupport::JSON.encode(roomId: room.id)
 
-      room.memberships.pluck(:user_id).each do |user_id|
-        ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(user_id), payload, coder: nil
-      end
+    room.memberships.pluck(:user_id).each do |user_id|
+      ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(user_id), payload, coder: nil
+    end
+  end
+
+  private
+    # The fanout is one publish per member, which in a big room takes far longer than the
+    # rest of posting a message, so it runs in a job rather than while the poster waits.
+    def broadcast_unread_room_later
+      Message::BroadcastUnreadRoomJob.perform_later(self)
     end
 end
diff --git a/test/channels/unread_rooms_channel_test.rb b/test/channels/unread_rooms_channel_test.rb
index 4444eb1..b3f36ed 100644
--- a/test/channels/unread_rooms_channel_test.rb
+++ b/test/channels/unread_rooms_channel_test.rb
@@ -16,7 +16,9 @@ class UnreadRoomsChannelTest < ActionCable::Channel::TestCase
     assert_not direct.users.include?(users(:jz)), "jz must be an outsider for this test to mean anything"
 
     broadcasts = capture_unread_broadcasts_for(users(:jz)) do
-      direct.messages.create!(body: "Private", creator: users(:kevin), client_message_id: "outsider").broadcast_create
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+        direct.messages.create!(body: "Private", creator: users(:kevin), client_message_id: "outsider").broadcast_create
+      end
     end
 
     assert_empty broadcasts
@@ -26,7 +28,9 @@ class UnreadRoomsChannelTest < ActionCable::Channel::TestCase
     direct = rooms(:bender_and_kevin)
 
     broadcasts = capture_unread_broadcasts_for(users(:kevin)) do
-      direct.messages.create!(body: "Private", creator: users(:bender), client_message_id: "member").broadcast_create
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+        direct.messages.create!(body: "Private", creator: users(:bender), client_message_id: "member").broadcast_create
+      end
     end
 
     assert_equal [ direct.id ], broadcasts.collect { |broadcast| broadcast["roomId"] }
diff --git a/test/controllers/messages_controller_test.rb b/test/controllers/messages_controller_test.rb
index 8088694..63266cb 100644
--- a/test/controllers/messages_controller_test.rb
+++ b/test/controllers/messages_controller_test.rb
@@ -60,18 +60,32 @@ class MessagesControllerTest < ActionDispatch::IntegrationTest
   test "creating a message broadcasts unread room to each member" do
     @room.users.each do |member|
       assert_broadcasts UnreadRoomsChannel.stream_name_for(member.id), 1 do
-        post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one #{member.id}", client_message_id: member.id } }
+        perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+          post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one #{member.id}", client_message_id: member.id } }
+        end
       end
     end
   end
 
+  test "creating a message leaves the unread fanout to a job" do
+    member = @room.users.excluding(users(:david)).first
+
+    assert_no_broadcasts UnreadRoomsChannel.stream_name_for(member.id) do
+      post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+    end
+
+    assert_enqueued_with job: Message::BroadcastUnreadRoomJob, args: [ Message.last ]
+  end
+
   test "creating a message doesn't broadcast unread room to non-members" do
     outsiders = User.where.not(id: @room.users.map(&:id))
     assert outsiders.any?, "need someone outside the room for this test to mean anything"
 
     outsiders.each do |outsider|
       assert_no_broadcasts UnreadRoomsChannel.stream_name_for(outsider.id) do
-        post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+        perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+          post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+        end
       end
     end
   end
diff --git a/test/system/unread_rooms_test.rb b/test/system/unread_rooms_test.rb
index 235e912..ea68ba0 100644
--- a/test/system/unread_rooms_test.rb
+++ b/test/system/unread_rooms_test.rb
@@ -15,8 +15,11 @@ class UnreadRoomsTest < ApplicationSystemTestCase
     using_session("Kevin") do
       sign_in "kevin@37signals.com"
       join_room designers_room
-      send_message("Hello!!")
-      send_message("Talking to myself?")
+
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+        send_message("Hello!!")
+        send_message("Talking to myself?")
+      end
     end
 
     assert_room_unread designers_room