KSP has several advantages over KAPT:
KAPT has to parse and resolve every type reference in order to generate Java stubs, whereas KSP resolves references on-demand. Delegating to javac also takes time.
Additionally, KSP’s incremental processing model has a finer granularity than just isolating and aggregating. It finds more opportunities to avoid reprocessing everything. Also, because KSP traces symbol resolutions dynamically, a change in a file is less likely to pollute other files and therefore the set of files to be reprocessed is smaller. This is not possible for KAPT because it delegates processing to javac.
KSP can process Java sources as well. The API is unified, meaning that when you parse a Java class and a Kotlin class you get a unified data structure in KSP.
KSP is currently in the beta phase, which means that the existing API is mostly fixed and unlikely to change. You are welcome to use it and we‘ll do our best to minimize the compatibility / migration issues for you. Please give it a try and let use know what’s needed and what doesn't work!