currently evaluating Rhino 6, and noticed the same slow .OBJ-Export Problem.
After some fooling around i noticed something. I almost exclusively work with a networked shared drive, and export is only slow and writing only a few kB per second (30-40 kBps) when i export the File to the drive.
If I export to my local drive instead, Rhino writes the file almost instantly.
I have noticed something similar in other programs (i think autodesk Meshmixer suffers from this aswell). It’s weird because normal Read/Write on the network drive is 50-100mBps, maybe it’s how the files themselves are written which is not suited for network transfer.
I guess writing the file in larger blocks or save to the userfolder in blocks first and then transfer to save location might fix this, but i can just guess.
Just so you know that might be another factor that causes slowdown, it might not necessarily be something in the Rhino files themselves.
Hope this helps someone. Cheers!