What you can do is create an "offline" version of your footage, and then once the edit is complete do an "online".
Pick a codec that is good to edit in, e.g all I-frame mpeg, or proRes / DNxHD or even HDV. The quality isn't actually too important, because before you make your master you delete the transcoded copies, and re-link to your camera footage. Then you can master off the original footage with zero quality loss.
If that doesn't work for you, then the first three codecs I mentioned will be an acceptable compromise. ProRes and DNxHD have the advantage of greater colour depth, at the cost of greater storage size, though since AVCHD is only 8-bit to start off with the advantage might be negligible. Whatever you choose, you want something that is all I-frame for editing.