toucanbuzz
No rule is inviolate
Although different speeds of XP advancement was talked about in development by Mike Mearls, it didn't make the cut, likely because DMs could incorporate this on their own if they wanted at their own rate.
Pathfinder (aka D&D 3.5+), did incorporate fast-medium-slow. When I ran games in that system, I preferred to slow it down so we could enjoy the ride. So, there's no reason you couldn't do that. It's just extra math (e.g. slow takes 50% more XP than the default rate) and consideration of how much treasure you're handing out (e.g. by slowing it down, are your players getting more magic items than would be expected, thereby making it tough to guess at appropriate level encounters?)
It's off-topic for your question, which isn't to convince you to use another method, but I've abandoned calculating XP because it's tons of math for little gain. I use a descriptive milestone (e.g. if your barony's capital reaches Town size and you resolve a major threat or 3 minor ones to your barony, you'll get a level).
Pathfinder (aka D&D 3.5+), did incorporate fast-medium-slow. When I ran games in that system, I preferred to slow it down so we could enjoy the ride. So, there's no reason you couldn't do that. It's just extra math (e.g. slow takes 50% more XP than the default rate) and consideration of how much treasure you're handing out (e.g. by slowing it down, are your players getting more magic items than would be expected, thereby making it tough to guess at appropriate level encounters?)
It's off-topic for your question, which isn't to convince you to use another method, but I've abandoned calculating XP because it's tons of math for little gain. I use a descriptive milestone (e.g. if your barony's capital reaches Town size and you resolve a major threat or 3 minor ones to your barony, you'll get a level).