I have used OpenCV, a native open-source image processing library. It comes with tons of built-in filters, but it is also very low-level. You extract pixels from a texture, send it over to OpenCV along with the steps you want, and off it goes.
Specifically I used it for QR-code-style image recognition, as well as some smoothing filters, but like I said, it only does the very lowest-level pixel manipulation and processing, but it is fast.
There is a Unity3D asset store package that nicely bundles up the OpenCV code and last I checked it was $100 or so, which will save you a TON of time trying to get it to build for all targets. Or you can download the source yourself and do whatever you need with it. Here’s a forum link on it:
And here’s their site:
https://enoxsoftware.com/opencvforunity/
It was very performant even back on 2014-2015 smart phones/tablets.