We have the same discussion internally from time to time. We also made a post about it here for speed comparisons with Rhino/GH, because a few colleagues were unhappy about the speed compared to Rhino.Inside.
Performance Checks Rhino/GH/Revit - Features & Ideas - Speckle Community
In the end, the time factor will be the most crucial one for the adoption of Speckle in order to succeed. Example: If sending Geometry from Grasshopper to Revit is drastically slower than Rhino.Inside, people will prefer Rhino.Inside … even if Speckle has in the overall process more benefits. If the overall file-based process is way shitier, Speckle will prevail. The same applies for Navisworks and Speckle vs. the nwc converter. Maybe some processes will only work file-based due to API limitations … but this is out of our hands. We cannot expect for a Trabi to drive like a Porsche … Trabi will be Trabi (aka Autodesk Revit API … and other vendors).
There are still some things the Specklers are able to optimize, but this needs time … as it is a big undertaking 
. For the platform to work on bigger projects, we need to have some patience. This platform has evolved drastically over the last years, therefore I’m fairly confident that they will manage to tune the gears a bit .