For me anyway, the SLDL and or Romanian DL gives me more all around athletic performance, strength and size. When doing them, always prefer doing forms of regular DL's in a trap/shrug Olympic style bar. More natural position , when lifting, when the hands are alone the sides, rather in front of the body. Takes leaning too far forward (a common cause of lower back problems when DL'ing) and hitting the knees (usually the reason people lean too far forward..avoiding shins or knees) mostly out of the movement.
With SLDL's I never go lower that the height of the '45's. Really no reason to. With the Romanian DL, which requires the lower back to be kept fixed and flat, you can not go much lower than a little below a 90 degree angle. RD's are one of the best hams exercises (when done right) there is.
Don't quite see any major difference to doing any form of DL'ing slow, fast or whatever. Though , if into sports, you may want to do them with quickness, like Olympic lifters do. Quickness training can adapt very well with power training. When reaching the max training poundage for DL's (or any other major lift) the weight will be going up at a slower & steady pace anyway. Good luck.
