I'm curious....do people really think about a round being 6 seconds?
I personally think that 6 seconds is too short. Remember that you not only get a move and an action in there, which can translate to several attacks, you also get a bonus action, a free object interaction and a reaction on top of that. You could open a door, run 30 feet, take four attacks with a longbow each with taking full aim, cast a bonus action spell after you did all this, and cast a shield spell as a reaction if the opponent tries to hit you, for example while you run past them. It seems too much too me.
One minute rounds were way too long. The handwavy explanations about feints and bobbying around waiting for the opening with only one attack that counts never did it for me. What if someone does not want to do any of that, and just wants to run away? They clear 60 feet running a full minute? Even with the old x4 speed multipliers from earlier editions for full run that ignores taking care of your surrounding that is a bit slow.
I think something like 10 seconds per round would be better.
I suspect the 6 seconds have been calibrated on running speed. That would be 10 rounds per minute, and at 30‘ speed and double move action you could run 600’ a minute, pretty much the speed unencumbered average humans run.
I think while light encumberance does not sliw you down in gsme terms, these average real-world human likely do not wear armor and 50 pounds of gear. I‘d think they‘d be a good bit slower with that.
The alternative, to keep with six second rounds could be to have an overall ceiling on actions, say not more than 3 a round. So you could move and attack and cast shiled as a reaction, but then could not take a free object interaction or bonus action that turn any more.
I think that would be less fun, as players live to do stuff. So you either suspend disbelief in the 6-second round, or you house rule it to be a bit longer and assume gear slows people down. In the latter case, I‘d probably also rule that a character just wearing a loincloth or such and no heavy or bulky gear can move with a 5‘ speed bonus.