Poor sluggish performance after upgrading to Yosemite
Beginning with Mavericks, Apple began using a different way of utilizing RAM called memory compression. That is why the used is so high.      

Wikipedia Virtual Memory Compression

Two things you can do are to remove the contents of the saved application state folder in your user library and remove the swap files in the /var/vm/ folder at the root of your drive. Once these have been removed, restart your computer. Let me know what you got going on after that!