I’d say SR13 and before for sure (well, it used to be slow again for a while due to poor handling of legacy materials, but @johnc made fixes for that…) So for a while it was good, but this slowness is quite recent, but not ‘just happened’. Sorry I can’t be more precise. It definitely worked in V6 well for a long time (since we switched to V6 last year).
Most of the work we do has to do with models that are orginally imported from other software (SKP, Revit) and Rhino is used to clean them up and built upon that/detail them further etc. These most often come imported as meshes, and very heavy scenes. Disorganized. Messy. Sometimes super slow since meshes don’t come joined. In order to be able to operate on these, we would use various combinations of join/splitdisjointmesh/extract mesh faces/ etc. Back and forth - since Rhino works super fast on big joined meshes. So we heavily rely on this being fast.
@pascal - I just saw your note - 6.10 might have been pre-JohnC fix. We switched to Rhino 6 around SR9 and helped to iron out a couple of issues, slow mesh joining was one of them, but it was due to some weird material buildup… I would say 6.11 to 6.13 was OK?