And has been too fast the last couple of editions.
This seems like a hard argument to justify, given level progression is pretty much entirely in the hands of the DM, and two groups, playing the same edition, are likely to progress at drastically different rates.
I mean, I saw 1E games where people leveled every session (thanks, gold = XP!), and I've seen 3E games where people went half-dozen sessions without leveling. The main differences were how the DM handed out XP, and how long the sessions were. Not everyone, is able to play a 4 hour+ session every week, for example.
Even looking at default rules, it's hard to see how 3E (for example) could be really seen as being "too fast", unless purely at 10+ levels, given it had a 13.3 encounters of equal CR expected per level, which is more "fairly easy/doable" encounters than it typically took to level in 2E, in my experience (until 10+).
As others have noted, it's probably the easiest thing to control, too - just give out more or less XP - you can simply multiply the XP you give out by a percentage to reduce it - i.e. half speed = just multiply all rewards by 0.5. Hardly need a rule for that, do you? But as Mistwell has noted, they are apparently going to discuss it.