Slow tempo can be very poweful in terms of letting melody evolve and progress throughout the composition however it is an appoach which will fail if the musician cannot understand how to build/phrase melody.
Good examples of the effective use of slow tempo would be godflesh, summoning, klaus schulze, assorted doom metal.
A bad example is perfectly illustrated in brian eno's 'music for aiports'. Melody doesn't evolve and the composition goes nowhere for long periods of time. He designed it that way and it's best use was to help prove this point.
Consistant medium to high speed tempo helps build intensity while reducing rhythm to the purpose of ambience, melodies become more repetitive and need to be immaculate in design i.e. jesu död, trans europe express, most of 'under a funeral moon' and 'transylvanian hunger'.
Using variations in rhythm to support the composition's progression. 'A blaze in the northern sky' comes to mind, my guess is it inherited this from death metal, but it's well paced. Constant time changes found all too often in death metal also lead nowhere.