Speed boots can give you additional movement points equal to their enchantment. There are a couple non-mutually-exclusive options for doing this:

• This only works for blessed speed boots.
• If the enchantment is negative, they subtract movement points, to a minimum of 1.
• To avoid the amount of movement points being predictable, the number of bonus movement points is slightly fuzzed by adding a random number from -1 to +1 each time it’s applied.
• The boots do not themselves provide any bonus movement, or provide a smaller one than they do now. The bonus from enchantment makes up the bulk of it.

Very fast speed from extrinsic sources such as speed boots is gently nerfed to be better than intrinsic speed, but worse than temporary sources.