Some people really undervalue the idea of having workers not feel pressured and having some privacy. You might get 10-20% more productivity out of them in the short term, but it raises stress a ton, destroys their mental health, and you probably lose that extra efficiency in burn out and depressed workers anyways. Over the long term it taints everything the company touches. It destroys morale, makes every interaction with a customer slightly worse, makes every work they do worse. It breeds resentment and a culture of dishonesty and irresponsibility.

Open layouts are terrible, humans are not designed to exist in an environment where they are constantly monitored. People have to take breaks sometimes, people have bad days sometimes. Everyone probably has a day at least once a year where they just really need a nap and a workday at once. The way you deal with this from a management perspective is you look at their average efficiency over longer periods of time and judge their worth based on that, but while preserving their humanity and autonomy. Humans are not meant to be drones, and good workers come from having happy and healthy workers. Give workers space, give them privacy, treat them well and they will care about your company, and if they don't then they are just not good people and never will be, so you should just let them go.

Efficiency also isn't everything. Life is not meant to be perfectly efficient. I understand capitalism makes this very difficult but real efficiency tends to be many micro optimizations over time without the actual workload increasing. You should expect that every worker is going to have some downtime everyday if they are working more than a few hours per day. This is not an issue, there are so many other ways to increase efficiency besides abusing workers.