You also have to remember that all of Horizons was planned to be released in 2016. However, it, unfortunately, took an extra year (something FDev obviously didn't foresee). If Horizons had been wrapped up and done in 2016 as Frontier had planned we'd currently be well into whatever is planned to drop in 2019.
It's why development feels so slow since we're technically a year behind what they previously planned. My guess is that they've been working on a lot of things behind the scenes the last few years and they're using Beyond to address the core of the game while they polish whatever it is they're dropping in 2019 (my bet is Space Legs, I'll eat a sock if it isn't).
If I'm wrong and we get nothing substantial in 2019, I will be severely disappointed and concerned with the future of the game. But until that happens (and I don't believe it will), I have faith.
While it was for very short period of time where FD thought of it as yearly release and they changed plans one or two months before Horizons 2.0 release, yes, it is how FD have worked before too - during first year there were lineup of smaller updates while other team worked on Horizons. And same is happening again - they got Beyond content mostly lined up with all ships, while team works on Q4 *and* premium DLC.
I think what contributes to feeling that it is slow is:
* New improvements to other gameplay than combat has been far and between. Q1 was first update to touch on trading and Q4 will be first one fundamentally changing how mining will work. While combat received lot of improvements, if you are not combat fan, then most likely you don't see them as huge improvements;
* Planetary gameplay has not seen much development. Again, very subjective, but also true that there are no new type of SRVs for example;
* No new fundamentally changing gameplay. Again, disclaimer, but even primitive atmospherics would feel very different;
That's a bit of curse of open world game. You can improve absolutely everything. And no one ever will be fully satisfied.