The story is heavy on plot and ploys, but none of those are interesting. The characters aren't likable or engaging. The male lead just acts like a golden retriever which is ridiculous as an actual personality.
I see too much of the view points of the villains to be annoyed at their erratic unsympathetic behavior. What is the point of showing me so much of thoughtless and annoying behavior ? I get he is an asshole, I don't need to see it again and again and again. Either make him interesting, or just give me the outline.
The FMC herself isn't that likable. While the Emperor is a much bigger asshole, the FMC shares some responsibility. She showed no affection to the Emperor for years because she was busy or she just can't be bothered. Starved for affection, he was seduced to a life of having partner that actually loved him. He did a lot of nonsense afterwards, but up to this point, there is no surprise their marriage failed. I'm pretty sure the Empress was more hurt by the fact he showed her disrespect and accused her unjustly of putting pressure on the mistress, than that he took a mistress, since she never loved him. She just thought of him as a good business partner for life. That is not a marriage.
The FMC then proceeds to have men fall for her, while she barely even acknowledges the fact they love her and is incapable of reciprocating affection to anything that isn't a small animal, a maid or the faceless masses. Maybe she will learn to love at some point, but I read over 120 chapters and it still hasn't really happened. It doesn't make sense she has such low EQ.
The former slave is just pitiful, naive and uneducated. She never experienced a parent's love, was abused her whole life. One can't claim she shouldn't have done whatever she could to free herself from slavery. Of course she is justified. She is clumsy in action and behavior, but to so say she is in the wrong because of that is just unfair. The story will have you believe the role of Empress is something like a top level clerical position, but it really isn't. It isn't about sitting and filling forms everyday, I don't get this Korean trope. If all she did was offer comfort and progeny to the Emperor, she is probably better as Empress.
Also, mind you the FMC is pro-slavery, permanent and inherited for a generation slavery. At no point in this story does she acknowledge her rival had a difficult upbringing or was in an unjust situation. She just felt disdain to her for being uneducated runaway slave. Given that, how good was she for the masses ? What percent of those masses are slaves ? I would guess the former slave is much more likely to try to abolish slavery. Maybe not immediately, but in 5 years after learning the ropes and maturing. The villain is presumably around 18-20 in the story while the FMC is around 25+.