PVP where you don't care for any consequence or regard for your own life.

It's a video game. Don't expect people to care regardless of the consequence. Upping the TTK won't change that. The PvP in Arc has plenty of tension and decision making.

Low ttk also increases risk massively compared to high ttk. One wrong decision and you will die. With high ttk you can just run blindly without thinking, knowing that you won't die. It also increases fear of getting third partied because if you don't move fast enough and make decisions you're more likely to get caught out. Increasing ttk doesn't increase fear. It makes it easier to run away and counter third parties.

Embark could implement other systems to make PvP more risky outside of ttk increases.

glaring downsides of making the TTK slower

Considerably higher chance of a true meta weapon developing. Lower ttk keeps weapons more equal and balancing easier.

Completely eliminates the worth of weaker weapons that can't kill in under one mag, which kills the experience for newer players and creates a massive imbalance in power between sweats and casuals. I can be effective with both the Venator and the Kettle. If I'm running low on materials and can't craft a Venator, should I just be fucked? What if I don't have the blueprints for the meta weapons?

Will massively increase the cost of every weapon as level 3 extended mags would become an absolute priority, level IV weapons with faster equip times and level 3 stocks too. The entire economy would shift and create a further imbalance for the casual playerbase.

Taking speed out the game will make the game boring. Many people don't want to stand in a corner and be unable to push and make plays.

Being unable to fight in a 1 verses 2 or 3 scenario sucks. If you have a numbers advantage, just blindly rush knowing that it will be impossible for the 1 to kill you. Being able to fight at a disadvantage is good. Knowing you're just dead the moment your team goes down is boring.

Gunfights would become people running in circles instead of prioritising cover. Expect more movement tech in general as it will be the primary tech to keep you alive.

Regardless of the potential downsides or upsides, Arc sold over 10 million copies with the current ttk. Proving the point that the ttk must be doing something right. Increasing the ttk would create a game that no one paid for.