I think Hickman was given a set period of time to tell his story, and he's using it to his advantage. A lot of writers don't have this luxury, and are forced to hit the ground running with their plots to immediately draw in readers, or face cancellation.
I thought it'd be interesting to compare where Hickman is now against other X-writers with long runs. Hickman is 21 issues into his X-men run. At this point:
The original silver age (Lee/Thomas) run had already introduced it's classic villains (Brotherhood, Juggernaut, Sentinels) and was starting to run out of steam- it's all aliens and robots for the next few years
Claremont had introduced a new team, killed Thunderbird, sent the X-men into space to repair the M'Kraan crystal, and was starting to reintroduce Magneto to the team at the 20 issue mark. Out of everything he'd done, only the M'Kraan crystal felt like a culmination of narrative instead of an introduction to be followed up on.
Lobdell wrapped up the most storylines out of the authors I considered, having overseen the entirety of the Mikhael Rasputin arc, the X-Cutioners Song, and Fatal Attractions during his first 20 issues. He also had an advantage in that his predecessors (Claremont, Lee, and Liefeld) had already set up the foundations for X-Cutioners Song and Fatal Attractions before they left, so Lobdell was able to play cleanup. Also, I think the Lobdell era shows the flaws in focusing too much on the present and not planning for the future, as a lot of his later foreshadowed plots suffered from poor execution, and as a whole his run is very erratic and uneven.
Morrison Destroyed Genosha, introduced Xorn, did some globetrotting with Phantomex and X-Corp, and as of 21 issues in, was in the middle of Riot at Xaviers at this point. Like Hickman, he seems to have been given a wide time period to work with, and I'd argue that he is also continuing to build at this point. The Cassandra Nova story would have been appeared to be resolved following the Imperium arc, but contemporaneous readers would have seen Ernst introduced at the school and wondered if it was really over.
Carey had finished three arcs ~20 issues in- the Supernova Arc, Blinded by the Light, and Messiah complex. Two of these three felt like they were seeding future stories, with Supernovas setting up a new antagonist group in the Children of the Atom and a new paradigm for Rogue following the Hecatomb arc, and Messiah Complex setting up the return of Mutants following M-Day.
Overall, I think in comparison to other writers, Hickman is progressing just fine. What's interesting about his plotting compared to other authors is that he eschews the standard arc format and instead progresses multiple plotlines simultaneously. Had he been working in the early 2000's "write for the trade" period, the growth of Krakoa, the introduction of the Arakki, the competition with humanity, and the return of the Children of the Vault would each be distinct storylines happening sequentially, but Hickman jumps back and forth between them. I like this organic unravelling of the plot, and I'm optimistic that it's going to produce a very cohesive work when all is said and done.