VTK 8.2.0 introduces a new module structure that allows individual compnents to be imported rather than the whole lot (which can take a while to load all the dlls). vtkplotlib 1.2.1 takes advantage of this new structure to reduce import time and make PyInstaller builds about 50MB smaller. If the installed VTK version is older that 8.2.0 then vtplotlib falls back to the original behaviour.
Due to VTK 8.2.0 PyPi releases not being out yet, this has only been tested with VTK 8.2.0 on Windows. Linux still runs OK with VTK 8.1.2.