What effect does turning off caching in all browsers have on SEO?
It slows down your page loading speed which can potentially be a negative ranking signal.
Turning off browser caching can be important for:
- Security (so that pages can't be found in browser cache)
- Accurate site stats (so that you can count each visit to a page)
I've worked with big sites that get lots of search engine referrals and don't allow page caching. It is very possible to have great rankings even without allowing browser caching.
Here's a one-word answer: BAD.
If no browser made use of a cache, then anytime a user requests a page, the browser would fetch it from the server and that fetch causes the server to run a bit slower because its an extra task it (the server) has to do along with other things that are running on it (such as email, etc). If millions of people requested pages this way within seconds, then the speed will be a-lot slower because the server could only handle so many requests at a time and the users in excess of that number will either wait a long time or receive some sort of error page (usually a timeout page).
Also, you should take a look at webpagetest.org.
And make a page with good caching, and make a page without caching, then run both those pages in the test and you will notice big differences. You'll even see timings of how long multiple requests take as well as an image-based slideshow of the page loading in action.