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I see many websites, seemingly arbitrarily adding the no cache meta tag on every page.

Like this:

<META HTTP-EQUIV="PRAGMA" CONTENT="NO-CACHE">
<META HTTP-EQUIV="CACHE-CONTROL" CONTENT="NO-CACHE">
<META HTTP-EQUIV="EXPIRES" CONTENT="-1">

What is it good for and why are they doing it?

Is it bad or good for performance?

Will it prevent http expires headers to function and ultimately slow down a website?

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2 Answers

up vote 2 down vote accepted

Caching speeds up repeat views and not first time visits so in terms of SEO value it means little since the first view time is the most important factor. Caching is good however because it means when people switch from page to page they are only loading the resources they need so this will boost user experience.

Using no-cache will disable the cache and <META HTTP-EQUIV="Expires" CONTENT="-1"> forces an immediate expiration on the file should the CMS or htaccess sets an expire.

Generally its not required and spoils user experience when setting up this way.

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These headers are used to discourage browsers or proxies from caching the page. For dynamically generated content the headers would be there to try and ensure site visitors are always hitting the server and so are always getting up-to-the minute content.

To answer your specific question, these headers may negatively affect performance because they may prevent caching, and caching is good for performance. But in practice they probably make little difference.

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Very few web pages themselves are explicitly cached. We all cache CSS/JS/images and often forget about the HTML. –  DisgruntledGoat Jun 10 '13 at 9:35
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