What version did you upgrade from?
Did you deploy an official release package or did you deploy your own build from source code? If you built from source did you make sure to deploy a release build and make sure <compilation debug="false" in Web.config? A debug build will be slower.
Are you running any custom features?
Are you using .NET 3.5 or .NET 4?
What kind of hosting do you have?
Are you seeing anything in the log under Administation > System Log
For example if you see constant logging of applicaiton start and end events it would usually mean not enough server memory is allocated to the site and that would kill performance.
I'm not aware of anything in the latest version that would degrade performance. Haven't seen that on my sites nor any other reports of it.