Korean dramas that don’t have any slow episodes at all?

Personally, I enjoy action series the most and that's because they're fast-paced and interesting. I'm not sure if it's your cup of tea or not, but I'm gonna leave some of my favorite here for you to enjoy :)

City Hunter (Korean, 2011)
- This is by far my best of the best, and it's from 6 years ago. City Hunter has the perfect combination of action and romance. The tone is not light, however, as it surrounds the idea of revenge, but it doesn't have the bloody/gore element that keeps non-action fans away. There's Lee Minho and Park Minyoung here as leads. Other supporting roles are also veteran actors as well. Each episode has a solid idea and storyline that keeps you on edge. It was never slow. I truly loved it. 

Healer (Korean, 2014)
- Now this is almost on the same level as City Hunter on my list. Honestly, the story was kind of similar, but they had their own unique charms. I personally enjoy City Hunter kind of story, so I definitely enjoyed Healer. If anything, I must say that Healer had a bit more lightweight romance element to it. The lead female was also Park Minyoung, the same as City Hunter, and I kind of thought that it would've been better with someone else although I'm her fan. The episodes were still fast-paced as well.

IRIS (Korean, 2009)
- This one is the heaviest out of all series I'm suggesting you. This is pure action blockbuster that I give a 10 out of 10 for every single aspect. Every episode was like a Hollywood movie. The cast was top of the country and the story was incredibly entertaining. It revolves around national secret security organizations. The very talented leads (just like in my other suggestions) are the ones that made each episode not boring. This is quite mature compared to other series, but the romance element was great as well.

You're All Surrounded (Korean, 2014)
- Now I honestly remember only the fact that this was a very funny yet promising action show. I haven't rewatched this one and don't remember much of the storyline, but I remember my feeling when I was watching it back then. If you do not like heavy action, this one is for you. The romance and comedy elements in this show is very great. I don't remember getting bored, so I can assure you that each episode was fast-paced. There's a solid story to this one also. It wasn't just a third-rate action comedy. The cast was great as well.

But if you've seen all of these, I agree with you that I'm still looking for a drama that doesn't drag each episode on for so long. I haven't found my favorite in such a long time. The spinoff and sequels of IRIS, such as IRIS II or Athena: Goddess of War, weren't as promising as the original as well. I guess the strong and fast-paced story was so overwhelming that it was enough in its own 20-episode length. Any more than that made me choke. 

Hope you enjoy :)

SIGNAL - the pacing of this series one of my fav
 

I agree with @WednesHwang on all of her recommendations and would like to add:

Whisper which just finished airing (I haven't seen the last two episodes) but there's a great quote from dramabeans "the average ratio of betrayals to episodes is about 17:1"

Also, even though it's only 4 episodes, Baek Hee Has Returned is brilliant and fast paced.  it's touching and hilarious.

Let's Eat never lets up.

Chief Kim has no slow episodes

Love me if you dare is fairly well paced but it doesn't change, there's no drop off

Oh My Ghostess maintains a quick pace and Witch's Romance has no slump

Also i disagree with @Mad-Clown_Rohit I found the first 5 episodes of signal excruciatingly slow, i almost dropped it.  I'm so glad I didn't because it's an excellent drama but the start is very slow

MrBanana

Oh My Ghostess
You Who Came From the Stars
Healer
City Hunter
You're All Surrounded
Signal

All these dramas get a little slow in the middle or end, these dramas are not fast at all, I've seen them

Incarnation of Money
Remember - War of the Sons
Defendant

All 3 dramas are kind of similar. Incarnation of Money and Remember are both about revenge. Incarnation was both serious and hilarious, but Remember on the other hand is more on the serious side. Remember and Defendant both has a crazy evil antagonist.

I watched all 3 without getting bored. They were intense from the beginning to the end :).