What camera are the AVCHD files from?
I edit AVCHD from my Sony camera without any problems on my Mid 2012 MacBook Pro, 2.3Ghz Intel Core i7, 16GB Memory, GeForce GT 350M, all the time. My source libraries are usually on an external USB 3.0 drive and I never create optimized media or proxies. I just edit the native AVCHD files as smooth as butter.
I always create Camera Archives from my camera cards and import from the archives. Never had any problems.
~jr
http://www.johnrofrano.com
http://www.vasstsoftware.com