Magento has a heavy structure so need to optimize so that website will run smoothly.
There are many steps to optimize the site in Magento
(1) Enable Magento caching(after admin login system/Cache Management).
(2) Merge Javascript file and Merge CSS file(after admin login system/ADVANCED/Developer).
(3) Compress images.
(4) Disable unneeded Magento modules(after admin login system/ADVANCED/Advanced).
(5) MySQL table optimization.
(6) Use Nginx or Litespeed webserver, not Apache.
(7) Use the Full Page Cache.
(8) Use a Dedicated Server.
(9) Use a Content Delivery Network (CDN) for parallel transfer of static content.
(10) Don’t use too many different external sources (for images, iframes, (twitter/Facebook)feeds, etc.)