Stianloader-remapper is an in-memory remapping engine for java bytecode. It uses objectweb's asm-tree library to operate directly on ClassNodes, resulting in no overhead for pipelines that already make use of them.
The main alternative to Stianloader-remapper is fabricmc's TinyRemapper, which uses the objectweb asm library and operates using ClassVisitors. However, it's main drawback is that it does not make use of in-memory remapping, meaning that it has to read from a file and write back to a file. If one chooses to apply layered mappings in a quick and dirty way by plainly remapping the contents repeatedly, then a severe performance loss can be expected due to I/O overhead caused by reading and writing to the same file.
TinyRemapper's strength is in being able to remap Mixins without the use of annotation processors (AP). Stianloader-remapper itself does not provide such functionality but this functionality will be provided as an add-on library in the future.