I have an admiral that has been parked at the L-cluster entry gate for 150 years fighting unbidden while i watched mostly AFK. In that time his fleet has killed several thousand unbidden ships while suffering the loss of only a single battleship (when 4 unbidden fleets jumped in simultaneously). He's only about to hit level 6 despite having +100% experience gain. His flagship, the titan ISS Majestic Carnage II, is only experienced rank.
The exp per combat for Admirals is way, way too low and individual ships need to get experience from more than just days in combat. It's confusing why this is even the case as their experience numbers haven't changed from 2.1 to 2.2 except for the addition of experience from fighting piracy. Perhaps the calculation is bugged.
Admirals are just the most extreme example, though. They nerfed governor experience from 3.5 per month plus experience for each building completed, tile cleared, pop grown, and active edicts to simply being a flat 5 per month. Scientists, Generals, and Rulers were already painfully slow before this patch and their numbers remain unchanged. Even with a 100% modifier to leader experience gain it still takes 250 years for a scientist on assist research to hit level 10. It takes 150 years in the same scenario for a ruler or governor to do so.
I'm not saying level 10 leaders should rain from the sky, but it shouldn't be basically impossible without massive leader life span modifiers. It also definitely shouldn't just be a straight function of time for 3 of the 5 leader types (once exploration is complete, anyways). This is especially true for governors because there's no way to level them up separately from their primary function like the other leader types.