Slower leveling is inherent in the system, simply be a jack of all trades.
I could do with some convincing about that...
The higher the skill is, the longer it takes to increase. But, the higher a skill is, the more it contributes to leveling up when you increase it.
A specialised character will have high skills that, when increased, contribute more to leveling up; but those skills will take longer to increase. A jack of all trades will have low skills that are faster to increase; but more of them are needed to level up. It's not clear to me that the rate of leveling up will be slower for the second type of character, because those two aspects which go into leveling up might balance each other out.
My impression of higher skills -> count more to leveling up was that it's designed to promote specialisation; and it's also not clear to me that a side-effect of the system is that specialised characters will level up faster. It's just that specialised characters will be stronger than generalist-type characters, where both have leveled up the same number of times.