I saw this tip posted on http://www.slrlounge.com/speed-adobe-lightrooms-performance/ and I can't believe the performance boost.
What I also can't believe is why LR doesn't swtch to using smart previews once you have them rendered instead of working directly with the raw file. WHY should I need to manually rename a folder to trick LR into using them?
Lightroom has been designed in that manner that it would read smart previews only in that scenarios when the Raw files are not available.
The main advantage of having smart previews is to allow editing while travelling when you do not have access to raw images or if your raw images are stored on external hard drive and you do not want to keep your external hard drive connected to your machine always.
Due to this reason you will have to manually rename or somehow trick Lightroom to not to use raw files if you want it to access smart previews.