diff --git a/luprex/cpp/core/lpxclient.cpp b/luprex/cpp/core/lpxclient.cpp
index ae862e46..e0b7744d 100644
--- a/luprex/cpp/core/lpxclient.cpp
+++ b/luprex/cpp/core/lpxclient.cpp
@@ -234,7 +234,7 @@ public:
     void receive_diff_from_server(StreamBuffer *sb) {
         world_to_synchronous();
         try {
-  	    DebugCollector dbc("");
+  	        DebugCollector dbc("");
             int64_t nactor = world_->patch_everything(sb, &dbc);
             if (nactor != actor_id_) change_actor_id(nactor);
             dbc.dump(stdostream());
diff --git a/luprex/cpp/core/lpxserver.cpp b/luprex/cpp/core/lpxserver.cpp
index 6a17e704..4e925ef3 100644
--- a/luprex/cpp/core/lpxserver.cpp
+++ b/luprex/cpp/core/lpxserver.cpp
@@ -14,6 +14,8 @@ public:
     int64_t actor_id_;
     SharedChannel channel_;
     UniqueWorld sync_;
+    double last_diff_;
+    bool async_diff_;
 };
 using UniqueClient = std::unique_ptr<Client>;
 using ClientVector = eng::vector<UniqueClient>;
@@ -29,6 +31,8 @@ public:
     HttpChannelVec http_server_channels_;
     int64_t admin_id_;
     Gui gui_;
+    int next_diff_chan_;
+    double next_tick_;
 
 public:
     virtual void event_init(std::string_view srcpk, int argc, char *argv[]) override {
@@ -57,6 +61,9 @@ public:
 
         // Export stuff to the graphics engine.
         set_visible_world_and_actor(master_.get(), admin_id_);
+
+        // for ticking.
+        next_tick_ = 0.0;
     }
 
     void do_luainvoke_command(const util::StringVec &words) {
@@ -165,11 +172,14 @@ public:
         client->channel_->out()->write_uint8(util::MSG_ACK);
         client->channel_->out()->write_uint32(0);
         client->sync_->invoke(inv);
-        send_diffs(client);
+        client->async_diff_ = true;
         return true;
     }
     
     virtual void event_update() override {
+        // Get the clock.
+        double clock = get_clock();
+
         // Execute any queued invocations.
         // We just feed these directly into the master model.
         eng::vector<UniqueInvocation> invocations = get_queued_invocations();
@@ -200,13 +210,14 @@ public:
                 client->actor_id_ = master_->create_login_actor();
                 // TODO: initialize the login actor on the master.
                 client->channel_ = std::move(chan);
+                client->async_diff_ = true;
+                client->last_diff_ = -100.0;
                 client->sync_.reset(new World(WORLD_TYPE_PREDICTIVE));
                 // This login actor is never used, it is just to preserve the invariant that
                 // the client model and the server synchronous model are identical.
                 client->sync_->create_login_actor();
                 clients_.emplace_back(client);
                 stdostream() << "New client: actor id=" << client->actor_id_ << std::endl;
-                send_diffs(clients_.back());
             } else if (chan->port() == 8080) {
                 HttpChannel htchan;
                 htchan.channel_ = chan;
@@ -214,6 +225,13 @@ public:
             }
         }
 
+        // If the clock has advanced far enough, tick the master model.
+        if (clock >= next_tick_) {
+            master_->invoke(Invocation(Invocation::KIND_TICK, 0, 0, ""));
+            next_tick_ += 1.0;
+            if (next_tick_ < clock + 0.3) next_tick_ = clock + 0.3;
+        }
+
         // Traverse all existing channels, process any communication.
         for (UniqueClient &client : clients_) {
             if (client->channel_->closed()) {
@@ -223,6 +241,15 @@ public:
             
             // Check for received invocations.
             while (handle_invocation(client));
+
+            // Possibly send a diff.
+            double diffdelay = 5.0;
+            if (client->async_diff_) diffdelay = 0.1;
+            if (clock >= client->last_diff_ + diffdelay) {
+                send_diffs(client);
+                client->last_diff_ = clock;
+                client->async_diff_ = false;
+            }
         }
         util::remove_nullptrs(clients_);