fix(sysmon): performance measurement (#5361)

src/core/lv_refr.c

@@ -328,11 +328,12 @@ void _lv_display_refr_timer(lv_timer_t * tmr)
 
     if(tmr) {
         disp_refr = tmr->user_data;
-        /**
-         * Ensure the timer does not run again automatically.
+        /* Ensure the timer does not run again automatically.
          * This is done before refreshing in case refreshing invalidates something else.
-         */
+         * However if the performance monitor is enabled keep the timer running to count the FPS.*/
+#if !(defined(LV_USE_PERF_MONITOR) && LV_USE_PERF_MONITOR)
         lv_timer_pause(tmr);
+#endif
     }
     else {
         disp_refr = lv_display_get_default();