All classics, that will get you hooked in the first episode. You are unlikely to skip any scenes in these ones :
What's Wrong With Secretary Kim - Romantic comedy
My Love From Another Star - Romantic comedy
Flower Of Evil - Thriller / Melodrama
Kingdom - Thriller
Descendants of The Sun - Romantic comedy / Melodrama
Crash Landing On You - Romantic comedy
Goblin - Romance
My Boss My Hero - Comedy
Then here are some that, while not necessarily slow and still pretty well-paced, might take a few more episodes to really get you hooked :
Love O2O - Romance
The Smile Has Left Your Eyes - Melodrama
Psychopath Diary - Comedy
It's Okay Not To Be Okay - Romance
Love Story In Harvard - Romance
I Hear Your Voice - Romance
Let's Fight Ghost - Romance
W - Romantic Comedy
Also, after watching hundreds of dramas, I've noticed that some dramas tend to be slower than others. So if you don't want a slow paced drama, I'll just advise you to stay away from the following kind of dramas in general: Healing dramas, Family dramas, Revenge dramas, dramas starting with the childhood of the protagonists, tragedy / serious dramas.