One thing about FF6's Esper system that caught my attention early on was that the learning speeds of various spells from various Espers didn't always follow a logical pattern. For example, Shiva and Ifrit teach the level 2 attack spell at 5x, while Ramuh teaches it at only 2x (making Maduin's 3x a faster way to get Bolt 2!) Poison is taught at 5x (Ramuh) while the much stronger Bio is available from Shoat at a whopping 8x! Doom is taught at 2x but X-zone at 5x.
The worst learning speed faux pas, IMO, is with Phoenix teaching Fire 3 at 3x and Cure 3 at 2x. The Phoenix's Cure 3 almost completely defeats the purpose of Starlet, and it doesn't make sense that Fire 3 can be learned three times as quickly as Ice 3 and Bolt 3 via Tritoch. (This last illogicality bothered me enough so that ever since I was a teenager, I made it a point to equip Phoenix ONLY on characters who had already completely mastered both Starlet and Tritoch. )
Have you ever pondered why there are these kind of inconsistencies in spell learning speeds? And more importantly, are there any ROM patches that change the learning speeds so they conform more closely to rational expectations?