Prisma has been available on Android for exactly a month now, but the app's developers haven't wasted time trying to improve it in the meantime. Shortly after the app's release, an update added a much-needed Save button, but there was still one major inconvenience to using the app: the wait times and the overloaded servers.
Prisma doesn't work like all other photo editing apps. Its effects aren't just regular filters à la Instagram, but they're built on neural networks that try to understand the original image and apply changes in a way that works specifically with them. So far, when you wanted to try a filter in the app, you had to wait until your photo was uploaded to Prisma's servers, the algorithms were run there to get your edit, and then the resulting image downloaded back again to your device.
It's why the app was so plagued with overloads and slow results when it first launched: the servers couldn't keep up with the demand. While the situation has gotten better as the initial flux of users has slowed down, it's still in the company's and user's best interest to offload that processing back to your phone. That way Prisma's servers wouldn't have to take the heat and results would show up much faster on your phone because they wouldn't require any uploading or downloading.
That might not be a long way off. According to a post made on the company's Facebook page, it looks like the offline feature is already live in the iOS app. The Android version will follow shortly (we've checked the most recent Android update - it's not there yet, but we'll let you know when it shows up). Only 16 filters will be processed offline at the beginning, but more will follow. Oh and it seems like Prisma is also working on video support, but that might be a way off from now.