Why my CodeIgniter Site Running Slow?

Hello!!
I have try all the passible solution but i am still facing my site running time is too bad.
I have tried

• Remove unnecessary libraries
  
• Use Autoload
  
• Reduce server response time
  
• I do not create configuration values
  
• Create a library for frequently used functions
  
• Remove index.php from the URL
  
• Compress content
  
• Use Sprites