A distilled Kottos will drop around x40 Speed Spheres, making it the quickest, cheapest, and most efficient way to farm these spheres. Speed Distillers can be purchased from most shops around Spira after your second visit to Bevelle. This includes the Calm Lands Shop, so you can buy Speed Distillers there (they cost 100 gil each) and then go farm Kottos for drops in the same area. Kottos is unlocked at the Monster Arena after capturing one of every fiend on the Mi’hen Highroad. The list is as follows:
Mi’ihen Fang Ipiria Floating Eye White Element Raldo Vouivre Bomb Dual Horn
What Can I Do With These Spheres?
Primarily, you can use them to activate Agility, Evasion, and Accuracy nodes on the Sphere Grid. Or alternatively, you can use some of them to customize the Distill Speed ability onto a weapon. Another alternative to that alternative is you can feed some Speed Spheres to your Aeons to teach them Extract Speed, Aim, and Reflex.
Distill Speed requires x2 Speed Spheres to customize onto a weapon Extract Speed (Aeon) requires x20 Speed Spheres Aim (Aeon) requires x5 Speed Spheres Reflex (Aeon) requires x10 Speed Spheres
And as is the case with most items, Rikku can mix up Speed Spheres to create some interesting results with her overdrive, such as: Super Mighty G (Chocobo Feather + Speed Sphere): Applies Haste, Protect, Shell, and Regen to the whole party. Chaos Grenade (Frag Grenade + Speed Sphere): Deals exceptionally large damage to all enemies, as well as inflicting Full Break, Slow, Poison, Darkness, Silence, and Sleep for eight turns. Final Phoenix (Phoenix Down + Speed Sphere): Revives all KO’d party members to full HP.
Are Speed Spheres Worth Farming?
Of course. And the Kottos method makes it so simple that you’d kind of be a fool not to. Speed Spheres are one of those things that you’re going to need a lot of, both in a regular no-frills playthrough, and for a max-stat endgame run.