In contrast to NWN, Arcanum's combat is at least fast and quickly gotten over with...
If so, that would appear to be because the enemies are even more easy and trashmobby.
No, it's because of the combat speed itself. If we take the exact same enemy and player stats (from a practical perspective: balance the player and enemy for D&D and Arcanum's system so they both end up with the same hit chance, HP, and damage in each system), it will be much quicker to dispose of that enemy in Arcanum than it is in NWN. The slower weapons in the game get one strike every two or three seconds. The fastest weapons strike once every half second or so. In NWN, there is one action every 6 seconds no matter which weapon you use. When you create a character focused on speed and a fast weapon, you can hit a trash mob rat ten times in five seconds. It's gonna be dead long before these five seconds have passed. In NWN, you haven't even performed your second strike yet.
NWN emulates D&D's turn based system by making "rounds" that last six seconds (as the D&D rules state), but these six seconds are supposed to be a rough abstraction for the turn based mode so people have a rough idea of how long a fight takes, and so spells with a duration in seconds or minutes or hours can be translated into turn times. When you apply the same system to real time, it ends up with fighters standing in front of each other, striking once, then not doing anything at all for 5 seconds before striking again... this only changes at high levels when you gain a second and later a third attack, but every other action than attacking is still limited to once every six seconds. It feels very artificial in a real time game. Real time games should not have underlying "rounds", they should be... you know...
real time. In Arcanum, every weapon has a speed factor. That determines how long its animation takes. A slow weapon has a slower attack animation, a fast weapon has a faster one. Once the animation is over, the next strike is performed immediately. No artificial downtime of two dudes just facing each other doing nothing.
That's why Arcanum is fast, and NWN is slow.
Morrowind and other action RPGs are a completely different ballpark, and not really comparable...
I do not see why not. It is relatively more comparable to NWN combat than comparing NWN to a party-based game. Like NWN, you are limited to a single character, with all the limitations that implies, except as a rule with even fewer tactical options and less reason to use what tactical options there are, since 99% of encounters are against easy trashmobs. I definitely agree that Morrowind would lose what virtues it had if transformed into an isometric game, but that is because the open-ended 3D environment is crucial to the exploration experience in that game. I do not see why the combat would not be about equally boring either way. Actually if you could walk away and just let your PC auto-attack trashmobs as is possible in IE games and NWN, it would be less tedious.
NWN lacks the tactical options of a party-based RPG, but it also lacks the quickness of combat in most single-character RPGs...
Maybe, but AFAIK, no one but you finds it especially slow, and if they have a problem with it, it is for other reasons. Theoretically, I suppose this could be subjected to an objective test, by running through encounters with a stopwatch or something like that, but in practice it would be difficult to adequately allow for all the relevant factors. E.g. if an NWN mob had more HP than a mob from Arcanum or whatever we are comparing it to, or the PC has less DPS, it is only to be expected that the fight will take longer. Obviously different tactics could make a difference too. At any rate, if it is slower, it is by such a limited degree that most players failed to notice, and makes up for that by offering more interesting build and tactical options than actiony RPGs typically do.
Again, if we were to apply the exact same stats to players and enemies in NWN, Arcanum, and Morrowind, that exact same enemy would take the exact same player much longer to defeat in NWN than in the other games. If an Arcanum mob had more HP than an NWN mob, it would still be quicker to dispose of... because Arcanum's combat speed is about six times faster than NWN's just due to the underlying system of how combat works. NWN with its six second RTwP rounds is by far one of the slowest RPGs I ever played. IIRC the "rounds" take the same amount of time in the Infinity Engine games, but those make up for it by giving you a party, which means more dudes to control during combat, so you have a lot more to do during those six seconds. NWN is single character and has more in common with Diablo than with BG or Gold Box (as you yourself admitted), so the slowness is a massive detriment to the experience, especially when playing as a fighter class whose tactics usually consist of hitting enemies with a weapon and occasionally quaffing a potion.
The only classes that offer interesting tactical options are casters, because they actually have other options than just standing there and hitting stuff. But even they are at the disadvantage of quickly getting swarmed by enemy mobs unless they bring a meatshield fighter companion... who may or may not do what he's supposed to because you can't directly control him.
And due to the six second rounds that run in the background, you don't even get to use most of the tactics available in other real time action RPGs. No kiting like in Diablo 2, where your sorceress can run away from melee enemies and toss fireballs at them as she runs. You can only toss one spell every six seconds, sorry. Trying to outmaneuver enemies and flank them as a ranged fighter is also not quite as viable, because you'll only get off one arrow before the enemy catches up with you. Remember, only one shot every six seconds. Running around doesn't take quite as long as performing an action, so having any advantage in distance or positioning will quickly be nullified. The only thing that would make tactics possible here is... yep, controllable companions. But they never do what you want them to do so their usefulness is limited. An archer could be viable if he gets a good position and has a melee fighter block off the approach. But when he's alone, and can only fire once every six seconds, and melee enemies can surround him without much issue... yeah. That's a problem.
Arcanum and Morrowind, on the other hand, were designed as flexible classless systems for single character adventures. I find both systems to offer a lot more variety and experimentation for character building than D&D does, which is naturally restricted by its class-based nature. A system designed for single character adventures is always going to be superior for those types of games than a system designed for party-based adventures...
One would expect so, though again design intentions do not always end up matching reality. 3E D&D with its great freedom for multi-classing actually comes very close to being just as flexible as a classless system. In any case, classless systems by their nature are going to have the same issue that you earlier identified (with reason) as the source of the problems with NWN combat: developers have to make encounters beatable by any reasonable build and as this is very difficult to do competently in the face of a wide variety of possible builds they tend to default to boring trashmob combat they can be confident anything will beat.
Either that, or you can design alternate approaches, make some encounters optional, etc. Morrowind doesn't do that, but Arcanum is pretty good at this (although not perfect), like allowing a diplomat to skip the Dredge, one of the hardest dungeons in the game.
One of the rare games that manages to keep encounters interesting and challenging while also being single character and offering a wide variety of builds is Underrail. Many different builds are effective, but you have to be careful not to turn yourself into a jack of all trades who becomes a master of none and will struggle against powerful enemies. And in some areas, the game allows for stealthy approaches so you don't have to engage with the enemies at all. Great game, and proof that the best single character games are those with a system designed and balanced around single character playthroughs. It still has some design decisions I'm not too fond of (fucking cooldowns), but overall it's one of the best single character isometric RPGs out there when it comes to variety of builds and tactics.