Be informed how fast your code is. On production. Realtime.
If you can not understand what users are saying, then you simply do not have the tools to analyze the problem. And users will go to your competitors. LOL :)
What do you know about 4000 models of mobile devices or thousands server types? They are different, but your code should work fast on each of them.
Customer behavior can be very strange and irrational. 1 request from 10 000 can block your server for 60sec because you don't know about that use case
Sintetic tests and profiling on developer environment very important, but it's a pice of all. On real hardware your code works a little different.
1. Write the code (PHP Sample)
1. PRFLR::init('hive', 'YourApiKey');
4. PRFLR::End('DB.AddItem', 'user');
2. After thousands of iterations web panel shows you the following information:
3. Use groupping, order and filters to get information about your code.
4. Find all the bottlenecks for 5 minutes and fix them.