I've used libvips a couple of years ago from golang via cgo. It was a bit rough to get started with it, but impressive once working.

It was quick enough to convert full-page pdf documents to black and white thermal printers (with scaling, didthering, autocrop and all that) in real time during an http request. Somewhar suprisingly, we also haven't had any memory leak issues, despite simply calling it dirctly in long-lived server processes.

Not sure about the state of the library now - my project has been discontinued - but would definitely check out libvips again if i'd have a need for an image manipulation library.