Using the Get EoX by Product ID method is the best way to speed it up. End of life data is by product, not serial number. So when the get EoX by SN is invoked, the API first has to convert the SN to a PID, then check to see if the PID is EOL.
That said, the average response time for all calls to the SN method was under 3 seconds over the last month (the PID method is under .5 seconds). So your 15 seconds is really an outlier. Not sure why that would be...
Regards,
Keith