Micha - thanks for sending through your scene. On my PC is takes 20 seconds per frame on my i7 laptop with 16GB RAM, Win10 and very slow hard disk. There are 1.2 million polygons on the scene, so I think 20 secs is quite reasonable to write all those vertices to disk in Alembic format. Your 60 secs per frame is either an exaggeration, or you are running on inadequate hardware. I also checked the code - and only Rhino meshes which has Live Geometry enabled get updated between frame (the whole scene still gets exported to the Alembic file though).
I will see if an Export Cancel button can be added to the next version. I will also add an option to export the Bongo animation to ORBX with only the camera track animated (ie. geometry will only be exported for the first first).
Paul