For previewing, make sure you're previewing in auto mode, not full res or anything.
When you start a new project, when you can choose the end res of the vid, try and lower your rendering quality a tad, may help a bit, but rendering is slow regardless. Painful, but seamless and PERFECT videos come out from it. Also, some transitions KILL rendering time, so try and get creative with stitching and stuff to reduce laggy trannies
Hope that helps a bit!