Ok - common problem - some help to be had but it is important to remember that preview is only and always will be a guide - a poor version of the final render - and it will ALWAYS slow down as you edit in a more complex fashion.
To speed up preview:
1. Set project properties to media properties when editing.
2. Have media on a defragged media drive seperate from system drive.
3. Lower size / quality as needed to speed up frame rate.
4. Never edit with motion blur on - add at end as this really slows down the edit.
But as i said preview will always be slow so...
1. Turn off processor hungry fx until the render - a good editor can intuit the effects.
2. Use ram pre render to check chunks with all fx on.
3. Use a desk top, not a lap top. Portables are not the toold for the job.
And finally - get over it - editing is and always be time consuming and slow.
Aslo - editors are designed to edit video from a camera - shock horror - and whilst most editors can handle weird compressed shit like mkv stuff they take loads more power to edit.
Power needed to edit - easy to hard....
DV - very smooth and fast and a big part of the reason i shoot DV still. Single core no prob.
HDV - About 4 times slower, dual core minimum.
AVCHD - hard drive cameras, four core needed to avoid frustration.
Divx / Xvid / WMV / MP4 type stuff - all likely to be found in the .mkv wrapper, four coress essential and even then expect weird glitches and slow clunky responses.