diff --git a/docs/source/reference/viewers.rst b/docs/source/reference/viewers.rst
index a42aff9e..3085d030 100644
--- a/docs/source/reference/viewers.rst
+++ b/docs/source/reference/viewers.rst
@@ -70,7 +70,7 @@ PyrenderViewer
   The viewer is initialized with specified resolution, update interval, and rendering flags. Key parameters include:
 
   - ``resolution``: Window size (default: ``(640, 480)``)
-  - ``update_interval``: Update frequency in seconds (default: ``1.0``)
+  - ``update_interval``: Update frequency in seconds (default: ``1 / 30``, i.e. 30 Hz)
   - ``enable_collision_toggle``: Enable collision/visual mesh switching (default: ``True``)
   - ``title``: Window title (default: ``'scikit-robot PyrenderViewer'``)
 
@@ -279,16 +279,17 @@ The following video demonstrates the interactive IK feature, where dragging the
     # Use viser viewer with visualize-urdf command (IK is enabled by default)
     skr visualize-urdf ~/.skrobot/pr2_description/pr2.urdf --viewer viser
 
-.. caution::
+.. note::
 
-  To speed up the rendering cycle in **TrimeshSceneViewer** and **PyrenderViewer**, adjust the ``update_interval`` to the reciprocal of the desired frequency. For example, to achieve updates at 30 Hz, set the ``update_interval`` to 1/30. This change will increase the frequency at which the ``redraw()`` function is called, making the rendering process faster.
+  Both **TrimeshSceneViewer** and **PyrenderViewer** update at 30 Hz by default (``update_interval=1/30``). The viewer only re-renders when the scene actually changes (e.g. after :func:`redraw`), so a static view stays cheap even at 30 Hz. The ``update_interval`` controls how often the ``redraw()`` request is polled: a smaller value gives a higher refresh rate (smoother interaction and animation) at the cost of more idle CPU, while a larger value lowers idle CPU usage.
 
   Example usage:
 
   .. code-block:: python
 
-    viewer = skrobot.viewers.TrimeshSceneViewer(resolution=(640, 480), update_interval=1.0/30)   # Set update interval for 30 Hz
-    viewer = skrobot.viewers.PyrenderViewer(resolution=(640, 480), update_interval=1.0/30)      # Same for PyrenderViewer
+    # 30 Hz is already the default; pass update_interval only to override it.
+    viewer = skrobot.viewers.TrimeshSceneViewer(resolution=(640, 480), update_interval=1.0/30)   # 30 Hz (default)
+    viewer = skrobot.viewers.PyrenderViewer(resolution=(640, 480), update_interval=1.0)           # 1 Hz, lower idle CPU
 
 
 Color Management
diff --git a/skrobot/viewers/_pyrender.py b/skrobot/viewers/_pyrender.py
index 6273ae2c..b751bada 100644
--- a/skrobot/viewers/_pyrender.py
+++ b/skrobot/viewers/_pyrender.py
@@ -99,7 +99,7 @@ class PyrenderViewer(pyrender.Viewer):
         The resolution of the viewer. Default is (640, 480).
     update_interval : float, optional
         The update interval (in seconds) for the viewer. Default is
-        1.0 seconds.
+        1 / 30 seconds (30 Hz).
     title : str, optional
         The title of the viewer window. Default is 'scikit-robot PyrenderViewer'.
     enable_collision_toggle : bool, optional
@@ -123,7 +123,7 @@ class PyrenderViewer(pyrender.Viewer):
     # Class variable to hold the single instance of the class.
     _instance = None
 
-    def __init__(self, resolution=None, update_interval=1.0,
+    def __init__(self, resolution=None, update_interval=1 / 30.0,
                  render_flags=None, title=None, enable_collision_toggle=True):
         if getattr(self, '_initialized', False):
             return
@@ -279,7 +279,10 @@ def on_draw(self):
 
         with self._render_lock:
             if not self._redraw:
-                super(PyrenderViewer, self).on_draw()
+                # Nothing changed since the last frame, so skip rendering
+                # entirely. Without this, the scene is re-rendered on every
+                # refresh tick (e.g. 30 times per second), which keeps the
+                # CPU busy even when the view is completely static.
                 return
             # apply latest angle-vector
             for link_id, (node, link) in self._visual_mesh_map.items():
diff --git a/skrobot/viewers/_trimesh.py b/skrobot/viewers/_trimesh.py
index ee60b20b..e0811a93 100644
--- a/skrobot/viewers/_trimesh.py
+++ b/skrobot/viewers/_trimesh.py
@@ -55,7 +55,7 @@ class TrimeshSceneViewer(trimesh.viewer.SceneViewer):
         The resolution of the viewer. Default is (640, 480).
     update_interval : float, optional
         The update interval (in seconds) for the viewer. Default is
-        1.0 seconds.
+        1 / 30 seconds (30 Hz).
     title : str, optional
         The title of the viewer window. Default is 'scikit-robot TrimeshSceneViewer'.
 
@@ -69,7 +69,7 @@ class TrimeshSceneViewer(trimesh.viewer.SceneViewer):
     _instance = None
     _version_warning_issued = False
 
-    def __init__(self, resolution=None, update_interval=1.0, title=None):
+    def __init__(self, resolution=None, update_interval=1 / 30.0, title=None):
         if getattr(self, '_initialized', False):
             return
         if resolution is None:
@@ -145,9 +145,10 @@ def on_update(self, dt):
 
     def on_draw(self):
         if not self._redraw:
-            with self.lock:
-                self._update_vertex_list()
-                super(TrimeshSceneViewer, self).on_draw()
+            # Nothing changed since the last frame, so skip rendering
+            # entirely. Without this, the scene is re-rendered on every
+            # refresh tick (e.g. 30 times per second), which keeps the
+            # CPU busy even when the view is completely static.
             return
 
         with self.lock:
@@ -263,6 +264,10 @@ def set_camera(self, *args, **kwargs):
                 )
 
     def save_image(self, file_obj):
+        # Force a redraw so the freshly rendered frame is captured. on_draw
+        # skips rendering when self._redraw is False, so without this the
+        # buffer read back below could contain a stale frame.
+        self._redraw = True
         self.switch_to()
         self.dispatch_events()
         self.dispatch_event('on_draw')