.625 XP/min - soylent (level 1), two festival currency (level 1, festival only)
.6 XP/min - glowing clawtips (level 2)
.57 XP/min - glass beaker (level 1), one festival currency (level 1, festival only)
.5 XP/min - reducing an item
Sizzling phosphorus is the most efficient recipe you can make until you reach level 9. Irradiated alchemical reductions are also good. Higher level apparel/familiars get you pretty good XP for your time and since they take a while to brew, you can set one brewing just before you go to sleep and let it brew while you’re gone.