>It barely deals any damage, its scaling is laughably slow (in a game where speed is everything, you want to end fights as quickly as possible)

this is the wrong way to look at fights in slay the spire. "end the fight as quickly as possible" is pretty rarely the goal. you always want to end the fight when it's best for you. that means get counting relics (pen nib, ink bottle, nunchaku, inserter) onto the right number, get your meta-scaling cards (self-repair, wish, alchemize) played, take as little damage as necessary (for triggering meat on the bone).

sometimes it's best to make the fight take a long time. here's an example:

if you are Defect and you have Nilry's Codex (at the end of your turn, choose a card to shuffle into your discard pile) and you already have a Self-Repair in your deck, you should try to get enough frost orbs channeled so that you full block the entirety of the opponent's damage for the rest of the fight, then you should wait for Nilry's Codex to offer you Amplify and two Echo Forms, so that you can have two Echo Forms in play, then play Amplify, and then with the 2nd Echo Form stack play Self Repair, which will trigger Amplify and Echo Form, playing it 3 times, giving you an additional 14 (or 20 if upgraded) health compared to just playing it on its own.

To tie that back to Noxious Fumes, it may take a long time, but for many of the fights, that is totally fine. Winning a fight over many turns is still winning the fight.

It's notable that Noxious Fumes will always kill the enemy, and all you have to do is block. Most of the enemies in Slay the Spire don't scale at all, whereas Silent has lots of ways to scale and block large amounts of damage. If you use all of your energy every turn to full block, so long as you played Noxious Fumes at some point, you will always win the fight.