Welcome to the forum.
Especially with AVCHD material, the computer is very, very important, as is the Project Preset chosen.
Can you give us much more info? This ARTICLE will give you tips.
If you have a red line at the top of your timeline you should render the previews by pressing [Enter] and wait for it to finish. When the line turns green the rendering has finished and you should see an improvement. As you add effects and transitions parts of the timeline will show red. Press [Enter] again to render those change.
Insanity is hereditary, you get it from your children
Read Harm on drive setup http://forums.adobe.com/thread/662972?tstart=0
- click the embedded picture in Harm's message to enlarge to reading size
Read Hunt on Partitions http://forums.adobe.com/thread/650708?tstart=0
A link with many ideas about computer setup http://forums.adobe.com/thread/436215?tstart=0