Disable sync reports when we know we won't need them.

Generating the sync report takes ~twice as long as
a normal tick, and occurs once every 3 ticks.

These reports record of all of the synced state
(separate to the sync hash, which is still calculated)
in order to generate the syncreport.log of the game
desyncs. This perf overhead is completely unnecessary
when we know that we won't have other syncreports to
compare against (singleplayer, replays).

Disabling report generation in these cases gives
us an easy 40% average tick-time win.

OpenRA.Game/Network/OrderManager.cs

@@ -52,6 +52,7 @@ namespace OpenRA.Network
 		public readonly ReadOnlyList<ChatLine> ChatCache;
 
 		bool disposed;
+		bool generateSyncReport = false;
 
 		void OutOfSync(int frame)
 		{
@@ -61,7 +62,12 @@ namespace OpenRA.Network
 
 		public void StartGame()
 		{
-			if (GameStarted) return;
+			if (GameStarted)
+				return;
+
+			// Generating sync reports is expensive, so only do it if we have
+			// other players to compare against if a desync did occur
+			generateSyncReport = !(Connection is ReplayConnection) && LobbyInfo.NonBotClients.Count() > 1;
 
 			NetFrameNumber = 1;
 			for (var i = NetFrameNumber; i <= FramesAhead; i++)
@@ -180,7 +186,8 @@ namespace OpenRA.Network
 
 			Connection.SendSync(NetFrameNumber, OrderIO.SerializeSync(World.SyncHash()));
 
-			syncReport.UpdateSyncReport();
+			if (generateSyncReport)
+				syncReport.UpdateSyncReport();
 
 			++NetFrameNumber;
 		}