After site growing in products we hit slowdown issues, after much worry and stress it turned out to be cache tag issues.
Uploading a single file and enabling it in local.xml then completely and utterly revolutionizes the speed of the site.
We are talking moving from 2 to 3 second category/product page to 400ms, and moving from 4 or 5 second+ for add to cart/checkout to around 800ms.
So my question is... what is the catch? Why is this not standard functionality in CE?
Not sure on this one, but Colin Mollenhour's Redis cache module has been part of Magento CE for some time. You find those pieces in lib/Cm and lib/Credis with copyright notice => Colin Mollenhour (http://colin.mollenhour.com)
Fabrizio Branca and Colin have both identified issues with Magento caching and contributed means of improving it.