Hello there, here's hoping I can answer your questions.
1) This is a rather odd way to phrase it, but the reason why it's taking you so long to kill other ships is due to the skill level of your gunners and the loadouts that your Captains choose. As I'm sure you understand, there are two types of hull- Permahull, and Hull Armour. Hull armour can be repaired and rebuilt endlessly, but Permahull cannot be repaired, and once it is reduced completely the ship dies. Onto my point about loadout: Different weapons deal different types of damage, and each type of damage is more or less effective against each component. The most effective type of damage against Hull Armour is Pierce, which is applied by the Gatling and other weapons. The most effective type of damage against Permahull is Explosive, which is dealt by, you guessed it, the Echidna. Hence, you need both weapons to take down a ship, as Pierce is poor against Permahull, while Explosive is terrible against Hull Armour.
With this, we can go back to the novice gunners- there is little point firing Flak shots at a ship before Hull Armour is down, as we can see. However, novice gunners do, and when the Gatling takes the Hull Armour down, the Flak is usually reloading, hence it misses the opportunity to get a full clip into the Permahull before the Hull Armour is rebuilt. Like this, it takes much longer to bring down ships. However, on top of that is the misunderstanding of the role of the Hwacha. The Manticore deals Shatter and Explosive damage. Shatter is good against components, such as guns or engines. As such, the Hwacha is in the 'Disabler' class of guns. However, the Hwacha is also heavily inaccurate. The problem with novice crews is that they view the Hwacha as the be-all and end-all. In fact, it will not kill anything until a Pierce weapon has taken the Hull Armour down, just disable ships. As such, Ships with Pierce weapons need to coordinate with Hwacha ships to drop the Hull Armour. Unprotected, Hwachas disintegrate Permahull. On a secondary note, the Hwacha is pretty much useless at anything beyond close range without Heavy Clip, as the shot spread is pretty huge.
Okay, question numero duo.
2) I agree, but it's more the Gat/Flak combo is simply the most efficient method of taking out a ship. The Artemis, Banshee and Heavy Flak are all unused simply because the Gat/Flak is better and more versatile. Also, Flame has been heavily nerfed (a single stack would disable a gun). Carronades are a tricky subject. In the hands of the right Captain and crew they are incredibly effective, especially against ships such as the Galleon. However, due to their recent nerf they are much harder to use, hence why I believe newer players view it as useless. Some weapons don't have any useful roles, but it's more about the dominance of the Gat/Flak. The Lumberjack, if used well, is devastating at long-range balloon popping, so I wouldn't underestimate it.
I hope this answers your questions, but if not reply here or catch me in-game.
EDIT: Gah! you put the edit in before I posted! Give me a sec
Done. Muzzle speed is the speed of the projectile as it leaves the weapon- reduced muzzle speed will quantify as quicker drop from shells. It's more like initial speed as it fires from the weapon.