If the cartridge is optimized for a shorter barrels

Ultimately, it's the physics of smokeless powder. Cartridges using fast-burning pistol powder, including .22LR and shotgun shells, are already making 80% of their total possible performance with 6" of barrel, and 90-95% with 12" of barrel. Some rifle calibers using M1 Carbine powder (H110 or equivalent), like .300 Blackout and some .357 Magnum loads, are also in this category.

(And yes, that means the M1 Carbine "enforcer" pistols are losing about 100 FPS by going from 18" to 12" of barrel.)

For rifles, you have 65% of your potential by 6", and 85% by 12". Most of the powder in these cartridges isn't finished burning by the time the projectile exits the barrel.

There are exactly two reasons for putting an excessively long barrel on your gun (outside of legal compliance): first, because you want a barrel weight and better sight radius with irons, which helps if you're trying to shoot down aerial targets, which in turn is why you find this on shotguns (a 14" full choke and a 30" full choke still produce the same pattern), and second, because you're issuing this rifle with the expectation you'll be charging at an enemy and your rifle + bayonet needs to be that long so you can stab the enemy before he stabs you.

Of course, longer barrels are also quieter than shorter barrels, which can be useful when suppressors are illegal if you don't want to pack hearing protection for whatever reason; this is an underrated benefit of PCCs as compared to pistols and rifles in more serious calibers.