Are you promoting only spell-related promotions?
Here in FF, XP gain is a more steady process, it is hower amplified by learning spells, and things like spell extension. Giving your adepts combat promotions will NOT contribute to this boost.
Also. Having 2 of a mana gives you the spell promotion free--which STILL gives a xp-rate-bonus. Getting 2 of alot of mana can make this process ~much~ faster. Abeit thats a little latter in the game. But with say, 2 of one mana(not unusual) Ive had no trouble getting adepts shooting up faster than I know what to do with them. *shrugs*
Now. This is with a non-arcane civ. I cant imagine arcane slows it down any ^.^' Potency increases the xp gain rate .70, a little over twice as much as learning a spell. Plus your spells are slightly harder to resist.
That said, if their innate gain and the arcane bonus is not enough. Sit em on a mage guild city (Another .25 rate bonus...), alchemy labs also proved ANOTHER .25 rate bonus.
With 2 spells(1 from free promo, 1 from first level courtsy of apprenticeship) you should have a rate around... 2.4 initially. Admittedly it will be going down gradually as your xp gains. But most levels, if not all you should be promoting a spell or spell extension. And hence increasing it by .3, note of course these rates go up a bit AGAIN when you upgrade to a mage. An extra .2 ^.^ allways nice.
Now that number may not be actuall gain/turn... but... its gona be pretty fast at that point. Mages-->Archmages shouldnt be much more effort at the very least ^.^'
Oh and if you are amurites and complaining. Stop. Build the Vave of Ancestors and... laugh evily. It gives a .50 rate bonus AND crap like you wouldnt belive. Not to mention their mages guild gives .35 rather than .25... and so on. XD...
*shrugs* Tho I must say, arcanes bonus does SEEM a little small. Although its where it counts. That extra chunk of gain-rate gets you over the humps much faster I think...
That said, in FF, there are ALOT more factors in adept-xp-gain ^.^'