Rendering is not so much affected by the format but like what jaegersing mentioned - the no. of FX you use. Other factors that will affect rendering speed are like the no. of tracks(audio & video) being rendered, the RAMs, PC heat and most important the NLE's way of rendering/ encoding etc
SP/ LP only affect your footages taken i.e. clarity/ quality, drop frames etc
For digital cameras like miniDV, the LP mode should not affect the video quality, but there are compatibility problems between cameras in LP mode, so you normally need to playback and capture using the same camera that took the shots.
Given the low cost of tapes, it is not really worth it to use LP mode, unless you need to shoot something that lasts more than an hour and you have no time to change tapes.