What is the Streaming Decryption setting in Google Drive?
Goggle Drive gives you some security options regarding your documents:


You can choose to encrypt all offline documents (that is, documents
stored on your device);
And can also choose to decrypt documents incrementally as they are
streamed to your phone, by selecting the Streaming Decryption, thus having just a small portion of the document decrypted when accessing it. Kinda like watching a compressed video.


Sadly enough couldn't find any official documentation to support my answer on Google, but here's some relevant links:

Access and Edit Files with Google Drive

Google Drive - View files offline in the Google Drive Android app