I happens with more then 10 units. Lets say 60 is a good number to start. Since I am just reinstalling it at the moment I canīt tell you exact numbers. I had 6 or 7 squads of melee and 6 or 7 of ranges tropps. By selecting them this lag happens. Its very heavy. I think when I selected lets say just 8 or so squads, it had lag , too. But not as much as with more.
Yes it stays as long as they are selected.
It happens when selecting them, not when giving them tasks (where it still lags, of course).