For example me, Im recording at 0,125x speed and more than 30 fps (in fraps and LFS is fps set to 50), than I give it 2x speed, and render it on 50 fps..
My slowmotion movies are littlebit laggy, low fps too, but with a new RAM I hope it will be ok... Now its smooth, but sometimes fps goes wrong (down about 2-5 fps and thats lag in finish movie)
Sorry for my english too :P