I've experienced this exact phenomenon in every edition I have played.
Even Mentzer's Companion, Master, and Immortals Rulesets took time.
What I think has changed most of all, is our sense of time. There is Pre Mobile phone sense of time, and then there is a Post Mobile phone sense of time.
The longest set piece battle I have ever ran for D&D took approximately 14 hours over 2 days. This involved, 10 players with 14th level characters, + NPC allies, + summoned creatures etc.
(It was also a NYE/ NY Day game..sober we could have shaved some time off)
In comparison, I've never played a game of Diplomacy that took less then 2 days.
In wargaming the joke that was always said was that the "recreation of the battle, took longer then the actual battle"
Slow cumbersome rules used to be a feature.
1e Shadowrun is the only game, that I found played very fast...and that ended once the Bodyguard Archetype was introduced.
Well, if you're running a small army, I can see why it takes you so long! All I can say is that most combats in 5E are over quicker than single rounds in D&D 4E at higher levels. We were lucky if I could get in 2 fights in a 6 hour game day, and those were just bog standard nothing too fancy fights (certainly not dozens of combatants per side).