There are two major factors that affect how fast and how far people advance in proficiency with any given skill set. These are natural talent, and practice. Natural talent is an affinity or the possession of some requisite trait that makes that skill set come more easily. Practice is the actual use of the skill set in question, either in a staged or a real-world setting.
There are of course other factors (psychological/affective for example) that impact people's learning outcomes. But in general, talent and practice are the big ones. And if my 25 years as a foreign language teacher have taught me anything, it's that practice is by far the more important of the two.
In game terms this effectively corresponds to earned XP for the practice element, and XP bonuses for the natural talent component. You can effectively place these on a set of X-Y axes to get the array of level advancement speeds possible.
Given that in-game, as in real life, practice carries more weight, advancement will be faster for people who have more practical experience (i.e. more earned XP), with the bonus for natural talent being a "value-added" component. Those with higher earned XP, who get more practical experience through adventuring, will advance faster regardless of modifiers, and those with low earned XP will advance slower regardless of modifiers. The fastest advance will be by people who have both high levels of practical experience (earned XP) plus natural talent (attribute bonus), and the slowest advance will be by people with both low levels of practical experience plus low levels of natural talent.
It is true that individuals with lots of natural talent will be underachieving slackers if they don't get the practice. I have seen this all to often in the classroom. Practice always has a greater impact than natural talent, and that is accounted for in-game in earned XP. If your character earns few XP (because the character isn't doing anything) then that character will by definition advance more slowly regardless of level of natural talent (which only gives you a bonus to what you earn through practice). But given equal amounts of practice, the person with an innate gift should always be more proficient than someone without the same level of natural ability.