For me, the time issue is ... not so straight forward or one sided.
If you control your boxes one at a time and also move all your alternate selves up to every fight, yes, it's going to take forever.
If, however, you main one character, clear everything, and then move up as needed to loot share with yourself, then imo its not that bad comparatively speaking (especially if you use 2 pieces of uncleansed LGS or something to kill and carry soulstones).
For myself, I sometimes run 6 at once. Regarding gear farming, I could solo run a quest 6 times. Or I can solo it once, only having to fight everything once, and run up 5 more times. This will take less time than running it 6 times, as the fighting only happens once. The end result is still 6 lootings of the end chest in both scenarios. So in that case, boxing is "faster" then solo if the end goal is farming loot. Obviously grouping with a full party of 6 real people is faster, but then you're dealing with people (and I've had people join farming runs, get the desired item, leave it in the chest unclaimed, and laugh about screwing over the one looking for that loot then leave. yes, seriously), and sometimes you don't want to deal with others.
For leveling, lets just take going from 20-30 (an epic reincarnation). Taking real life/work/etc into account, when I'm soloing my main only, it takes me on average 1 week to do this. When I box all 6 of my accounts (after an epic reinc on all of them), it takes me about 2 weeks to get them all back to cap while running them at the same time. I do not use software, so I move all of me around one at a time. Given that if I was soloing each one it would take 1 week each (6 weeks total), while 2 weeks is twice as long for my main, it's 1/3rd the time for all of them (of course, when leveling, I don't run them to the end for loot unless it's worth it for sentient exp or a chance for reaper loot upgrades). So that extra time is what moving to each quest and running through wildernesses without teleports adds (for me at least). So overall when considering all of them together, it's faster. But when compared to what one does if they are ONLY concerned about their main character, it is slower.
Of course, I like my boxes to be build well enough that if I needed or wanted to main as them for a while, I could. Now, gearing up so many characters is a fair amount of effort, so I understand that is not for everyone.