The least 'CPU intensive' way I've found of editing AVCHD, is to turn it into Canopus HQ. Takes about 3 x real time to transcode using a 2.4GHz P4. Files are about 3 times the size of DV files.
These are intraframe files, which you can edit using Edius.
OK,the files are a bit big, but because they are 'intraframe' they don't need that much computer 'grunt' to play well.
You can use Canopus HQ outside Edius, and there is a free AVCHD2HQ converter, which you can get from the Canopus (Grass Valley) site, which uses the Canopus codec to create Canopus HQ files.
The cheapest way of getting the 'legit' Canopus HQ codec is to buy Edius Neo - which you can trial - although it's not very difficult to find 'freeware' versions of the Canopus codec online. Whether they're supposed to be there as 'freeware'?.......
Only way I've found of dealing with AVCHD on an ancient P4!