Can someone explain to me why 5 seconds stun on DS is too strong

-first you must clarify correctly what is your point when making a post

and about your post, most of dead by daylight developers only test the changes playing internally so they never get to experience a real matchmaking in solo Q or the mischief of random players, so if they implement something is based on the test they made internally or just make the changes in the game and never try them out before releasing patches, this is why most perks or killers are extremely disbalance in the value they have in game, also the recent changes buffing survivors made tunneling and slugging a necesite for killers to even get one kill so 5 seconds could make a chase of 1 minute in to a 3 minute chase being the equivalent of 3/5 gens chase, but behavior get almost no complains from killer sides since most of the tunneling/slugging "problems" is they own fault for his game mechanics from 2016 that always incentivise focusing survivors and just now it is an "issue" for people complaining too much

Because sometimes you're not tunneling when it happens to you. It's a minute long timer post-unhook, which, accounting for the time it takes for someone to come in and unhook, is plenty of time to have won a chase and hooked someone else. Especially on snowball killers like Oni, Twins, and Plague.

To start, yes, it does matter for every single killer. Even comparing bottom tiers, a 5 second stun will mean less to a Skull Merchant than it will to a Hag, due to the fact that she can gain haste and the survivor can go broken during the stun. That 5 second stun may mean multiple blown traps for the Hag when she cannot teleport to punish them, or may mean wasting precious seconds of power for Michael. For killers like Kaneki and Nurse, the difference between the two is that Nurse now needs to possibly waste a fatigue to catch up, while Kaneki loses absolutely nothing by catching up to the survivor. Reasonings like this can be found across the entire roster, which is why it's balanced to 4 seconds currently (and could possibly be pushed down to 3 seconds to shrink the gap even further, but as it currently is, it's probably fine).

Secondly, DS is not a get out of chase free card, it is a perk that is meant to keep you safe until you can get back to the objective. To use a common example from the last few days; if you are blind to the trap under your foot, am I, the Trapper who's the next loop over, meant to ignore you because you might have DS? No, it's in my best interest to actually eat that DS so you cannot use it again and remain unsafe for the rest of the match. You're still safe to get back to the objective due to the 4 second stun, but I also know now that you do not have DS to use again (and if you weren't the Obsession, it also reveals that the Obsession has either Mettle of Man, Sole Survivor, or DS as well) and therefore will not be safe until healed moving forward. I can even choose to chase you again and make you unsafe again, and you can choose to walk back into a different trap!

If you're getting two downs within 60 seconds of a hook, that one extra second of stun ain't stopping you.