Heh. I don't think "rapid" is the right word. Last year they had said they were hoping for monthly-ish patches, but that went by the wayside pretty fast around Alpha 2.3 or so, when they began integrating the backend persistence systems.
The Persistent Universe Alpha is currently at 2.6.x. It currently includes one "limited" area of space around a gas giant in one specific solar system. (Note that "limited" here means that it's only a fraction of the planned final solar system, but thanks to CIG's 64-bit map size technology, it's legitimately several hundred million KM^3.) There's a few space stations, several quests, and some limited economy. All fully playable, but gameplay-wise, it's still mostly at the "tech demo" level in a lot of ways.
The "holy grail" version that we fans have been waiting for since last fall is Alpha 3.0. The /r/starcitizen subreddit has a ton more info on what it's supposed to include, but off the top of my head: a larger chunk of the Stanton solar system; several moons or planets that will be 100% "see it -> fly there -> land there"; the first iterations of several professions; and a whole lot more.
The current production schedule for Alpha 3.0 is at https://robertsspaceindustries.com/schedule-report . They originally projected it would come out around June 29, but just pushed it back a couple weeks. Realistically, I would expect them to bump it back into August. Once that's out, they'll probably get out another couple good-sized updates later in the year.
So, definitely not "rapid", but given the scope of what they're doing, not surprising. (Besides, my laptop actually can't even handle playing the game right now, so I've just been watching the development progress. I'll probably upgrade later this year sometime after Alpha 3.0 comes out.)