Slow? hm, it's a two cour, if a servant died every episode we would need 50 of them
The episode continued character built-up, it's a huge cast... 15-20 characters... and despite some posters' expectation both this and F/SN are novels, so action non-stop is not the objective... yet because of the transition to anime it can be complementing very well the plot and characterization, like in Rakkyou.
Now for Maiya X Kiritsugu, it's very common, while being in love and/or married to have an affair, even a long one. Especially in older times marriage was rarely confused with love, and usually retained it's original purpose... ownership, it's a rather modern phenomenon to try to bind legally and/or morally the people you love
