chore(scripts): warn if we don't respect the target FPS (#2986)

src/lerobot/scripts/lerobot_record.py

@@ -398,7 +398,14 @@ def record_loop(
             )
 
         dt_s = time.perf_counter() - start_loop_t
-        precise_sleep(max(1 / fps - dt_s, 0.0))
+
+        sleep_time_s: float = 1 / fps - dt_s
+        if sleep_time_s < 0:
+            logging.warning(
+                f"Record loop is running slower ({1 / dt_s:.1f} Hz) than the target FPS ({fps} Hz). Dataset frames might be dropped and robot control might be unstable. Common causes are: 1) Camera FPS not keeping up 2) Policy inference taking too long 3) CPU starvation"
+            )
+
+        precise_sleep(max(sleep_time_s, 0.0))
 
         timestamp = time.perf_counter() - start_episode_t