You will only face higher execution time for function when it is called after a certain period of inactivity. If invoked continuously, you will not face the slowness. You can check the access logs for your function and check the average execution time of your function. 

We would suggest you to enable Application Performance Monitoring which will help you get an idea on your function execution timing along with component calls etc. You can find the help documentation for the same here. Do check on the same and let us know if you further have any queries.