audio_core: Only perform audio stretching if below full speed. (#7201)

src/core/perf_stats.h

@@ -42,6 +42,8 @@ public:
 
     Results GetAndResetStats(std::chrono::microseconds current_system_time_us);
 
+    Results GetLastStats();
+
     /**
      * Returns the arithmetic mean of all frametime values stored in the performance history.
      */
@@ -82,6 +84,9 @@ private:
     Clock::time_point frame_begin = reset_point;
     /// Total visible duration (including frame-limiting, etc.) of the previous system frame
     Clock::duration previous_frame_length = Clock::duration::zero();
+
+    /// Last recorded performance statistics.
+    Results last_stats;
 };
 
 class FrameLimiter {