I think the difference is here there really aren’t that many people swimming at the same time, or at least I try to avoid those times. Besides those 50m days I rarely see enough people at the pool to need 2 per lane let alone worrying about getting 3+ per lane.
As far as being to slow for a lane, the only pool I’ve swam at that had markers for slow, medium, fast, was at the Iowa State University pool. I doubt most people here, that weren’t swimmers, would even realize that you should get in a lane with someone swimming close to your same speed.
Trust me I get what you’re saying, but reality here in at least most the pools I’ve been in, a third person joining is very rare, theres hardly any need to even have to worry about it. On those days where it is, like the 50m day, I circle swim.