[changelog] Add breaking change note about CPU frequency

changelog/2025.6.0.rst

@@ -151,6 +151,11 @@ focus on three key areas:
 These optimizations are part of an ongoing effort to make ESPHome more efficient and reliable. More performance
 improvements are planned for future releases.
 
+Arduino Platform Default CPU Frequency
+---------------------------------------------
+
+The default CPU frequency for ESP32 is now 160MHz for both ESP-IDF and Arduino (was 240MHz for Arduino). This can
+be configured in the ``esp32`` block using the ``cpu_frequency`` option. See :doc:`/components/esp32`.
 
 BME68x BSEC2 Arduino Independence
 ---------------------------------
@@ -217,6 +222,7 @@ Breaking Changes
 - Optimize Component and Application state storage from uint32_t to uint8_t :esphomepr:`9082` by :ghuser:`bdraco` (breaking-change)
 - Reduce entity memory usage by eliminating field shadowing and bit-packing :esphomepr:`9076` by :ghuser:`bdraco` (breaking-change)
 - Optimize Application ``area_`` from ``std::string`` to ``const char*`` :esphomepr:`9085` by :ghuser:`bdraco` (breaking-change)
+- The default CPU frequency for ESP32 is now 160MHz for both ESP-IDF and Arduino (was 240MHz for Arduino) :esphomepr:`8542` by :ghuser:`clydebarrow` (breaking-change)
 
 Beta Changes
 ^^^^^^^^^^^^