Depends on the game. If you have a regular grid, attached can be better. Infinite runner for example. But if more organic results are needed, then modular is better.
So you'd say it's common practice to finish a dynamesh blockout of a creature for example, and then manually retopo a base mesh from it to subdivide + sculpt in zbrush?
Yeah, that's why I used myself as an example. I shouldn't even have gotten this email, but if they consider an anatomy doodle to be 'Adult', this could be a problem.
Both Unreal and Unity have modes for examining this, and other bottlenecks. For example: https://docs.unity3d.com/Manual/OptimizingGraphicsPerformance.html Are you writing your own renderer?