I'm using a subdomain multisite installation with 9 sites.
For caching I tried W3TC, which worked great for the frontend, but slows the backend extreme. I deinstalled W3TC, which speeds up the network dashboard extreme, but the other sites are still very slow.
Right now I'm using Cachify with APC, but the sites are still not that fast. Sometimes the pending times for loading (Chrome Network Tools) are about 24 seconds :-(
Deactivating and Reactivating the plugins did not show any improvements.
For server architecture I'm using nginx, php5-fpm and apc.
Do you have any advice?