core: perf_stats: GetLastFrameTimeScale should reflect target framerate.

src/core/perf_stats.cpp

@@ -108,13 +108,15 @@ PerfStatsResults PerfStats::GetAndResetStats(microseconds current_system_time_us
     system_frames = 0;
     game_frames = 0;
 
+    target_fps = static_cast<u32>(results.game_fps / results.emulation_speed);
+
     return results;
 }
 
 double PerfStats::GetLastFrameTimeScale() {
     std::lock_guard lock{object_mutex};
 
-    constexpr double FRAME_LENGTH = 1.0 / 60;
+    double FRAME_LENGTH = 1.0 / target_fps;
     return duration_cast<DoubleSecs>(previous_frame_length).count() / FRAME_LENGTH;
 }
 

src/core/perf_stats.h

@@ -81,6 +81,8 @@ 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();
+
+    u32 target_fps{60};
 };
 
 class FrameLimiter {