Hi.
Performing a test in the program SAP_TIMER_DEMO, it is possible to see that there is no process running in SM50 while the timer is active. In SM04 the memory consumption also remains the same while the timer is running. So I could not see any performance issue.
As per the OSS note below seems that SAP also use this functionality in their standard application:
1606887 - Massman: Auto-refresh
Regards,
Felipe