Claude Code Desktop: WindowServer at ~47% CPU while a response streams (deep CoreAnimation layer tree re-walked at 120 Hz) #94003

Summary

While the Claude desktop app generates a response, macOS WindowServer runs at about 47% of one CPU core. The app window is otherwise cheap: with the app idle and the same window visible, WindowServer sits at 3-6%.

A stack sample of WindowServer shows the time goes into re-walking the Claude window's CoreAnimation layer tree, which nests about 50 levels deep, once per display refresh (120 Hz on this machine).

Environment

• Claude desktop 1.52386.3
• macOS 26.6.2 (25G83)
• MacBook Pro, Apple M3 Pro, 14-core GPU
• Built-in Liquid Retina XDR display, 1512x982 points @ 3024x1964, 120 Hz

Reproduce

1. Open the Claude desktop app with one window visible and nothing else on the space.
2. Record WindowServer CPU every 2 seconds.
3. Send a prompt that produces a long reply.
4. Compare the CPU while the reply streams against the CPU when the app is idle.

Measurement

One continuous 2-minute trace. Nothing else changed. No other app window was on screen.

   2s   48.5%   previous reply still streaming
   6s   34.0%
   8s    8.0%   streaming stops
  12s    3.0%
  ...
  54s    3.0%   idle
  58s    3.0%
  60s    5.0%
  62s   37.0%   new reply starts streaming
  64s   50.0%
  73s   50.5%
  87s   48.0%
 101s   48.0%
 105s   27.5%   reply ends
 111s    2.5%
 121s    3.0%   idle

Idle: 3-6%. Streaming: 41-51%.

Stack sample

sudo sample WindowServer 10. All other WindowServer threads are idle; the render-server thread is blocked in mach_msg for 6440 of 6472 samples. All the time is on the main thread:

CGXRunOneServicesPass
  post_port_data
    non_coalesced_timer_handler
      run_timer_pass
        update_display_callback(void*, double)              2545
          CGXUpdateDisplay                                  2544
            WS::Updater::UpdateDisplays                     2050
              WS::Updater::prepare_coreanimation            1893
                ca_prepare_begin_window_update(CGXWindow*)  1657
                  WSSessionConnectionPerformWithPluginOwner 1654
                    CARenderUpdateAddContext2               1487
                      CA::Render::Update::add_context        802
                        CA::Render::Updater::prepare_layer0    \
                        CA::Render::Updater::prepare_sublayer0 / ~50 levels of recursion
                        CA::Render::Updater::prepare_portal

One window accounts for this. The ca_prepare_begin_window_update branch with 1657 samples is the Claude window. Every other ca_prepare_begin_window_update branch in the sample has 20 samples or fewer.

Attribution

• Hiding the Claude window (Command+H) during streaming takes WindowServer from 47% to 1.5%. Restoring it returns to 47%.
• Hiding every other application on the space changes nothing. Nine applications were eliminated one group at a time.
• A second Electron application was on screen for one round and contributed nothing measurable. This is specific to the Claude window.
• A shorter conversation does not help. A fresh session with a two-message transcript measured 51.2%.
• A smaller window costs less, consistent with fewer layers in the tree.

Ruled out

Scaled display resolution (tested at native 1512x982 @ 3024x1964), video playback in another app, HID event rate (2.3 events/s), accessibility zoom and color filters (all off), EDR (headroom 1.0, inactive), Stage Manager (off), external displays (none), GPU saturation (10% device utilization), WindowServer log errors (none in 20 minutes).

Expected

A streaming text animation should not require WindowServer to re-walk a 50-level layer tree at 120 Hz. Flattening the animated region into fewer composited layers, or confining the animation to its own layer, should remove most of this cost.