NetVips is a wrapper for the libvips image processing library. It's a bit like Magick.NET, but typically 8x faster. It's API-stable, supports around 300 image-processing operations, and a good range of image formats is supported.
NuGet package: https://www.nuget.org/packages/NetVips
There's a benchmark here: https://github.com/kleisauke/net-vips/tree/master/tests/NetVips.Benchmarks
Formatted docs, including a tutorial and full API: https://kleisauke.github.io/net-vips/
Blog post: https://jcupitt.github.io/libvips/2018/04/10/libvips-for-dot-net.html