I'm gonna agree with the other posters here. I don't think reducing detonation time is necessarily good for people who play against him, despite what I'd like. It changes his traps to an in-combat alternative spell, since the arm-time is so short. Making the detonation time shorter would make them far more dangerous if, say, you were actively kiting someone and throwing traps behind you.
Even champions without boots can proceed to clear my traps by just triggering them and walking away. Once they get boots they can walk THROUGH them completely from edge to far edge, without taking damage. I support the idea that the slow should be more severe when the traps trigger to make them a consideration for my enemies even when my W is down. I also wish that the detonation damage actually filled the whole slow zone, I don't know if it's just me but I feel like if I catch someone right on the edge of my trap it hits them for bupkiss.
Alternatively, I feel like the cooldown on W is way long, and while it makes great picks with backup...it's not usually the settling factor if jhin is ever by himself fighting or dueling unless you're running something like deathfire's touch. Even with a cdr build his W rests at roughly a 15 second cooldown. I kinda want some kind of cooldown reduction on his W in his kit that relies on him successfully landing trap damage (ether from his passive death blossoms or his manually placed traps). If it were a 2 second cooldown reduction, for instance, I feel like that would help his utility a lot and honestly improve his aesthetic for me.