From what I remember, Roshi wasn't exactly one of Master Mutaito's best students. Neither was Shen either. And then by the time Piccolo showed up he and Shen were Mutaito's only surviving students and probably the only two humans alive that not only knew that ki was a thing but also have a clue on how to use it. The spending 50 years making the technique could very well be the result of Roshi's own incomplete training as well as his own lack of personal talent. Considering how of his previous set of students(Gohan and the Ox King),if the Kamehameha wave is so easy to learn, why was it that only Gohan had picked it up? And why was Roshi so surprised that so many others were picking it up so easily? There is very much a reason why Roshi called Krillin, Yamcha and Tien gifted martial artist prodigies after seeing how easily they learned it, they pretty much are and much more then either he or Shen ever were too.
1. Roshi certainly, at least at the time that Goku traveled back in time and met him, but that was probably in large part due to his laziness and lechery; it was the events of his meeting with Goku that seemed to motivate him to take the martial arts a bit more seriously. Shen, though, actually did seem to have some clout as one of Mutaito's students, considering he had flunkies and all. At the very least, he wasn't as much a loser as Roshi, since even the dumbest bully wouldn't act like he did if he didn't think he had enough weight to throw around. Plus, we don't really see any of Mutaito's other students fight very much.
2. According to Roshi's flashbacks, he and Shen fought Piccolo's armies alongside the entirety of Mutaito's school, and by the end only the two of them were left. They might not necessarily have been Mutaito's
best students, but neither of them is a coward, so I really doubt they survived when everyone around them fell because they ran and hid or used their comrades as human shields. They had to at least have been among his
better students to live that long.
3. Mutaito may have been the greatest martial artist of his time, but it's not like he fared very well against King Piccolo. He got thrown around just as easily as Tien, whom Roshi could fight fairly even against despite being past his prime. He might not have reached or surpassed Mutaito's level (it's hard to say, since Mutaito's only shown battle was getting his ass kicked against King Piccolo and somehow *cough*plothole*cough* getting the best of a post-King Piccolo Goku), but he didn't completely lack for talent. It's honestly unfair to compare him to his students, who all had opportunities that he really didn't. I'm not just talking about training with Kami and everything above that, either, I'm also talking about strong rivals (i.e. Goku, who as a saiyan is just kind of cheating) who could constantly push him to improve himself more and more. Roshi seems like he kind of stagnated after becoming the strongest in the world, which was probably part of why he thought he needed to do the Jackie Chun thing in the first place.
4. The Ox King is probably unimpressive as one of Roshi's students at least partly because he didn't devote himself to the martial arts the way all of his other students did. He settled down in some mountain kingdom and raised a family; the biggest fights he was in after that point was probably chasing off bandits and wild animals, not really something that keeps a fighter of that level at peak condition. He's never shown doing all that much in the way of training, and in the one or two fights he was ever in he mostly relied on his size and strength. I'd say he was probably never one of Roshi's better students anyway.
5. Yeah, Roshi's students are all 1 in a billion prodigies while he likely isn't (though I'd argue he must have had some level of talent that set him above others), but the point I was making was that it's almost certainly easier to figure out the Kamehameha by watching it than it was for Roshi to develop it from scratch. It's still incredibly impressive the way Goku and Tien figured it out just from seeing it once, and Krillin and Yamcha were only a little slower, but it could very well also be partly for the reasons I gave above. Since it's never stated exactly why it took Roshi 50 years to make the technique, we're free to draw our own conclusions, and I prefer the one that doesn't make him a total joke compared to his students. He's already got enough to be depressed about.