AFAIK transcode-as-a-service vendors already offer real-time/streaming transcoding, so it's over as soon as you finish uploading the file.
Even a real-time hardware encoder isn't enough for the target use-case here, where the user already has the video in the cloud and wants to apply a transformation (like an edit or filter) and then re-encode the results. ExCamera is trying to make that as interactive/quick as possible. "Real-time" on an hourlong video would still mean waiting like an hour for each operation.
Ultimately, YouTube uses chunks of 128 frames each because it's too expensive to put key frames much more frequently than that, and other transcoding services will face a similar limitation. ExCamera's purely functional encoder lets it use chunks smaller than the interval between key frames, so the effective degree of parallelism can be a lot higher.
(Obviously there's a lot of overhead from using a software codec -- we think hardware codecs should support a "save/restore state" interface too. I think we do have to prototype first in software to demonstrate the benefits before hardware people will take us seriously.)