Actually Ferrothorn and escavalier are the slowest fully evolved Pokemon, but here's some things that can help all Pokemon
Abilities: sand rush, swift swim, chlorophyll, speed boost, quick feet, steadfast, motor drive moody unburden
-Speed Boost raises the holders speed by one level each turn
-Chlorophyll doubles speed in sun
-Swift Swim doubles speed in rain
-Sand Rush doubles speed in sand
-Quick Feet boosts speed when statused
-Motor Drive boosts speed when hit with an electric type move
-Unburden boosts speed when the item is lost
-Moody may boost speed at the end of each turn
-Steadfast boosts speed when the holder flinches
Natures: timid, hasty, jolly, naive
-Timid boosts speed and lowers attack
-Hasty boosts speed and lowers defense
-Jolly boosts speed and lowers special attack
-Naive boosts speed and lowers special attack
Effort Values and Individual values
-Having 252 evs and 31 IVs will maximize speed
Items
-Choice Scarf boosts speed by 1.5x, but restricts the user to one move
-X-speed raises speed one level, but like all stat boosts disappears after switching
-Quick Powder (only for ditto)
-Quick Claw prioritizes the users first move, as does custap berry
-Salac berry boosts speed one stage at 25% hp
Moves
-Boost by one level:
--dragon dance
--silver wind
--ancient power
--*ominous wind
--quiver dance
--flame charge
-Boost by two levels:
--auto maize
--shift gear
--agility
--rock polish
--she'll smash
--tailwind (it doubles the whole teams speed for four turns)