Magento website loading speed is very slow

1.Combine JS and CSS files

(i) Magento admin -> System Configuration -> Developer -> 

Under “Javascript Settings”, 

change “Merge Javascript Files” to YES.
(ii)Magento admin -> System Configuration -> Developer -> 

Under “CSS Settings”,

Change “Merge CSS Files” to YES

Note: after made this change, sometimes it show effect on entire website. In this case, please revert back the settings to "No"
2. Use Magento’s Compilation feature. It’s reported to give you a 25%-50% performance boost: System > Config. > Tools > Compilation.
Note: after made this change, sometimes it show effect on entire website. In this case, follow this url

http://tejabhagavan.blogspot.in/2016/01/when-i-enabling-compilation-in-magento.html

3.Enable flat catalog :

Go to System > Configuration > Catalog >Frontend,
 change Use Flat Catalog Category to YES. If desired, under Frontend, change Use Flat Catalog Product to YES. Clear the cache.
4. Enabled magento caching
5. MySQL Query caching
6. Enable Gzip Compression
7. Disable any unused modules
8. Disable the Magento log
9. Optimise your images