diff --git a/python/mujoco/viewer.py b/python/mujoco/viewer.py
index 48ad9405..61520240 100644
--- a/python/mujoco/viewer.py
+++ b/python/mujoco/viewer.py
@@ -300,7 +300,7 @@ def _physics_loop(simulate: _Simulate, loader: Optional[_InternalLoaderType]):
 
           # Run mj_forward, to update rendering and joint sliders.
           mujoco.mj_forward(m, d)
-          synccpu = time.time()
+          simulate.speed_changed = True
 
 
 def _launch_internal(
diff --git a/simulate/main.cc b/simulate/main.cc
index 5ccc3ee6..71aad17d 100644
--- a/simulate/main.cc
+++ b/simulate/main.cc
@@ -400,7 +400,7 @@ void PhysicsLoop(mj::Simulate& sim) {
         else {
           // run mj_forward, to update rendering and joint sliders
           mj_forward(m, d);
-          syncCPU = mj::Simulate::Clock::now();
+          sim.speed_changed = true;
         }
       }
     }  // release std::lock_guard<std::mutex>