> I read somewhere that the less seams in a model, the faster.
They could be referring to UV unwrapping speed (don’t know if that’s true or not) or they could be referring to the fact that seamed vertices need to be transformed twice in most engines (which is also true of sharp edges, anything with discontinuous vertex data). The second probably isn’t a big issue, it just means, don’t make every triangle its own island even if you’re just baking a procedural texture.
For follow quads unwrapping, you’re still going to have seams as regards this double-transformation, even if you haven’t marked seams. Any place that a single vert is doubled on the UV map, that’s a seam, that’s a place where the vertex will need to be transformed more than once.
Follow active quads seams relatively straightforwardly: if it’s already UV mapped a quad, it doesn’t do it again. So the precise location of how it seams depends on the order in which it maps quads. I’m afraid I don’t know which exact order it chooses.
However, if you do seam a mesh and use follow active quads to generate UV, follow active quads will respect those seams. I always seam my meshes, even if unwrapping with follow active quads, because I want control of where those seams are going to be. Meshes that are designed for follow active quads are straightforward to seam: select, ctrl rt click select shortest path, mark seam. If they’re not simple enough to seam in that way, they’re probably not good candidates for follow active quads.