I believe you Oofta, to be clear, and you know I'm a 4E fan.
I think taking an hour per round is probably towards "extremely unlucky" end of the scale in terms of PC/monster numbers, but taking WAY TOO FREAKIN' LONG? That was common once you got into the teen-levels in 4E.
So I believe you and sympathize with your position. I loved 4E, but we changed to Dungeon World when the PCs were all like level 14 or 15 or so and a combat was taking 1-2 hours, or 3 hours, instead of, say, 30-40 minutes. We were also working hard at speeding things up - not criticising you here btw, we were going above and beyond - we have really well-used initiative cards, we used a whiteboard to track stuff AND a battlemap AND counters for stuff - everything we could come up with to make it go faster.
And it still bogged down in the mid-teens. It was as you were saying, all the complex interactions that did it. Reactions leading to Interrupts leading to Immediate Actions. People moving around in initiative. Working out the full impacts of some highly complex ability involving damage, CC, and area denial, and so on.
Now we saw somewhat similar issues with 3.XE/PF, but with a different root cause - i.e. when people got to the 5-10 range and Feats and PrCs and obscure rules and stuff started to pile up, and the at like 7+ magic got more and more powerful and complex to adjudicate, and we saw longer combat rounds at lower levels than 4E. But we never saw rounds as very long as we ultimately saw in 4E.
The worst we ever saw was Champions: The New Millennium (a version of Champions which ran on R Talsorian's FUZION engine), where 3 minutes of in-game interaction and combat between like 4 PC heroes and 5 NPC villains took over 5 hours to resolve. We never played it again lol.