You will find most people playing videogames care more about gameplay than plot, and when the "takes 20 hours to get going" complaint is levied, it's usually at gameplay, not plot. Very few people would complain that the plot isn't super dramatic if they're actually, you know, having fun playing.
The two other frequent use cases for that sentence are 1) when those 20 hours consist mostly of non-gameplay (in a game genre expected to contain a more balanced gameplay ratio), or 2) if it's a game genre with very little gameplay, like a visual novel. In the first case the complaint is still 100% justified (frankly, if you want to make an RPG or action game that is mostly custcenes or dialogue, go write a damn book); in the latter it's much more subjective and case-by-case, and there are some games that probably couldn't even work any other way (Steins;Gate is one of those cases; the world building and clue peppering is so subtle that you think nothing is happening and it all feels quite pointless, until it pulls the rug out from you...).
tl;dr: 20 hours of non-gameplay =/= 20 hours of bad gameplay =/= 20 hours of bad story =/= 20 hours of slow story.