Monsters which do not move at normal speed generally move “slowly” (-10 to speed), “fairly quickly” (+5), “quickly” (+10), “very quickly” (+20) or “incredibly quickly” (+30). (It will surprise nobody that Morgoth is one of the few monsters in the last category.) This is further adjusted by the fact that any non-unique monster may have a random adjustment from (-2) to (+2) to its own speed.
Generally, (+10) is exactly double normal speed, and (-10) exactly half. (+20) is about three times normal speed, but after that there is less noticeable improvement as speed goes higher – for instance, (+30) is not quite four times normal speed, and higher values than this are largely irrelevant. The player may find items which can be worn or wielded that provide speed bonuses: these may include boots of speed, rings of speed and a few very rare artifacts. Boots will provide a random 1d10 to speed: rings of speed may be bigger than that – generally the best that the player will get is two just over (+10), but individual rings of up to (+23) speed have been known.
Separate from the question of permanent speed (as determined by the player’s speed items and the monster’s natural speed) is that of temporary speed. The player may cast a spell of haste-self, or use a potion, staff or rod of speed or use an artifact activation to speed him temporarily: or a monster may cast a haste-self spell, or be affected by another monster “shrieking for help” or the player reading a scroll of aggravate monster. In all cases, (+10) speed is added temporarily to the affected monster or player. Using two or more sources of temporary speed is cumulative only in duration – one cannot get from normal speed to (+20) using a potion and a spell of speed. Spells of temporary slowing (including monsters breathing inertia or gravity) are handled the same way, with exactly (-10) being subtracted from the player or monster’s speed temporarily, for the duration of the spell or breath’s effect.