What is caching and why does it make a site faster?

Caching means keeping a result you already calculated so you do not have to do it again. It is the most effective speed lever for a website.

Page caching

Your server builds the page once and then serves the saved version. A site that had to query the database on every visit now does so once per period.

Browser caching

Images, stylesheets and scripts are stored at the visitor's end. The second page they open loads noticeably faster because half of it was already there.

Object caching

Query results kept in memory. Useful for sites with a lot of dynamic content where page caching does not apply.

The downside

A change is not visible immediately. This causes half the support tickets on cached sites: somebody edits a text and does not see it change.

What you do

Clear the cache after a change and know where the button is. And do not cache personalised pages — basket, account, forms.