Many stillwater fly fishers considering working flies in water deeper than 10 feet think a fast sinking fly line of type 3 density or greater is the way to go.  At first glance flies desending at better than a 3 second per inch pace makes sense.  If the goal was just to get the fly down near the bottom fast sinking lines would seem to be a wise strategy.  However, the reality is that fly fishers need to present the pattern in that magic zone 1 to 3 feet above the bottom weeds and debris.  To be successful imitating the myriad of stillwater food sources stillwater retrieves must be slow and varied.  As a result utilizing a slow sinking intermediate or Stillwater line makes sense.  Intermediate or Stillwater lines sink anywhere from 1-2 inches per second, almost half the rate of faster sinking lines such as a Type III.  The aim is to select a fly line with a sink rate that does not over power the presentation.  Leech, scud or dragon nymph patterns that remain in the strike zone longer stand a greater chance of being intercepted by foraging trout.  Fast sinking lines result in fouled flies and frustration.  Using a quick descent line results in a faster paced retrieve in order to keep the fly out of trouble.  Large wary trout are suspicious of any food sources bouncing along at an unnatural pace.  Remember, the slower the retrieve the more life-like the presentation.  Personally, painstaking slow retrieves are a favored tactic when fishing is tough and trout seem unwilling to come to the fly.