Okay, spoiler time...
Turn back now if you don't want to be spoiled....
Okay, that might be enough. So, let's rewind.
Riftan was born a serf and his step-dad worked really hard and used his connections to let Riftan become a blacksmith apprentice (let's estimate he's 8-10.) This allowed him into the castle grounds, where occasionally he would see Max playing alone with her dog. There are two interactions where they see each other and exchange a horseshoe for a flower crown by placing them where the other would see it but they don't talk to each other. Their third interaction is when a monster comes into the grounds and attacks Max. Her dog dies and she kicks the monster for taking away her only friend, to which it scratches her. Riftan saves her and takes her up to the main castle so she can be treated and he is rewarded with a pouch of silver.
We still don't know who Riftan's dad is but someone who knew his father came by and gave him a dagger that didn't belong to serfs like them. His mom thinks he's dead and hangs herself. This means she can't have a proper funeral so in the night, his step-dad has him carry her corpse on his back into the woods and they use farming tools to make it look like she was mauled by animals instead, thus letting her have a normal burial and not damning their entire household. Riftan decides to leave and sneaks onto a caravan.
Fast forward and Riftan is 16 when he meets Ruth for the first time. They're mercinaries and since he has nothing to live for, Riftan is a throw caution to the wind, crazy kamikaze soldier. He and Ruth end up being the only ones alive when they end up facing a dragon. Riftan gets injured and to take his mind off the pain, Ruth tells him to think of his happiest memories and he can amplify those feelings as he heals him to take him away from the pain. The only good memory he has is of Max. And because it felt so nice, he would occasionally ask Ruth to do it again. It starts to become like an addiction and Ruth tells him he can't keep doing it because it will drive him mad, not knowing the same memory has been replaying each time. Thus, Ruth technically brainwashed Riftan into seeing Max as this perfect person and he will have a much harder time separating himself from this idea.
Circling back to his mom: because of what he and his step-dad had to do (he only carried her and step-dad sent him away a bit to do the mauling,) every time a woman touches him, it reminds him of her corpse on his back. So thanks to the brainwashing, Max ends up being the only woman who doesn't do this to him. Because he killed the dragon, he is rewarded by the king (emperor?) with his lands and he returns to his home. He asks Max's father for her hand and of course, he says he will never be good enough and makes fun of him. He doesn't get to talk to her or even make eye contact because, of course, she's still being beaten and can't look at anyone. He thinks she feels the same way. While he is there, he finds the new son of his step-dad, has him set up a private meeting, and gives his step-dad some money he earned.
Well well, a war/another dragon to defeat (I am fuzzy at the moment but this is when he travels with the princess) comes up and Max's dad doesn't want to participate. He has to go or send his "son" to go in his stead. Suddenly, Riftan is good enough for his daughter but because he is a despicable b*stard and doesn't believe for a second his daughter can keep a man like him, he threatens him to marry his daughter by imprisoning his step-dad for having money from seemingly nowhere. His new son had to run to Riftan and he went to the castle. Worried about his past of being a serf coming out second only to his step-dad being unjustly imprisoned, he agrees to being married. Of course, everyone tells Max the first time hurts and a man like him could beat her to death if she didn't satisfy him so she's scared out of her mind when they do the obligatory consummation the day before he leaves to fight. Then, her dad beats her like Riftan was supposed to make her six months pregnant overnight or some crap and continues to beat her the entire time he's gone.
And to go past where we are now in the timeline, yes, Max has a miscarry when she goes to try and help Riftan defeat some monsters down the line...after being buried in an avalanche and healing herself so she doesn't die. She didn't know she was pregnant so it was early and it goes all around the nobility like she's a useless woman because she can't keep a baby. Riftan felt that maybe she wanted to be home for a bit after the incident so he sends her home. Obviously, she takes that as she has failed him as his wife and he's returning her, which is also how her father interprets it. Later, when Riftan comes back to get her, they have to break into the castle and he and his knights (including the one that has been giving her a hard time) see her chained up and being whipped across the back by her father. Riftan beats him close to an inch of his life and he's healed but I think I remember them saying he ends up having a permanent stutter (could be making up that last bit because it sounds like poetic justice.) All the knights who witnessed this all become very protective of Max as well.
Of course, his blue blood being spilled "without reason" his terms for Riftan to be punished. Since Max's stepsister, married to the king (emperor?), was psychologically abused by their father, she has no hard feelings for what happened to him but can't intervene. The punishment would have been severe but Max offers to go to the wizard's tower for three years in his place. Riftan is stubborn to the end, saying he won't wait for her if she goes (he does though, for the same reasons explained earlier.) She gets to come out of her shell more and make friends at the tower. She specializes in earth and water magic, which gives her the affectionate nickname of Mud. She also does something forbidden but can hide it so no one else knows or she can never leave the tower.
I actually haven't read up to where Riftan beats the living hell out of her father so that and everything following is spoiled from someone else. Because like you, I felt this story was getting to be a slog at times. I find it more tolerable knowing WHY Riftan isn't able to let Max do anything but be a well-kept woman. It was also good to know that he does find out what her father did to her and that the knights now understand why she never came to the territory while he was away. The fact that it's taking forever is both a strong and weak point. The slow burn is building other plot points around to pay off in the long run. I would occasionally give this a glance to see where it's at, look at the pretty art, and lie in wait for the good bits.
Riftan needs to calm down and stop treating a grown woman like a child. I understand where he is coming from but...