They used to run them in parallel, but the troops took a lot longer to train. If they made them run in parallel again, they would increase training times again. The reason they switched them to training in series was to make it easier to manage. And it is. It's WAY easier.
By giving you individual training times, you should get an indication of how long a given army would take. For example, look at a Lavaloon attack, once now, once in 2015:
3 Hounds, 10 minions, 26 balloons
2 Rage, 2 Freeze, 3 Haste
Today:
3 Hounds = 15 minutes
10 Minions = 3 minutes
26 Balloons = 13 minutes
Troop training takes 31 minutes
2 Rage = 12 minutes
2 Freeze= 12 minutes
3 Haste = 9 minutes
Spell brewing takes 33 minutes
So, heroes not included, your entire army takes 33 minutes to train. Now, let's go back to 2015, when troops took longer, but trained in parallel...
3 Hounds (can cook 2 at a time) = 90 minutes
10 Minions (can cook with third hound) = 7.5 minutes (cooks in tandem with third hound, effectively 0 minutes)
26 Balloons (can cook 4 at a time = 56 minutes (cooks in tandem with lava hound, effectively 0 minutes)
Troop training takes 90 minutes
2 Rage = 60 minutes (Were Rage and Freeze 30 minutes at this point? Or were they still 45? Let's go 30 for argument's sake...)
2 Freeze = 60 minutes
3 Haste = 30 minutes (can brew in tandem with elixir spells, effectively 0 minutes)
Spell brewing takes 120 minutes
Regardless of heroes, your army takes 2 full hours to train
See the difference? The reason troops train so quickly is because they train in series now. It's an improvement. Believe me or don't. We don't want parallel training back
