KB means "Knowledge Base". KB Builder gathers all what it can in your sources and builds the so called "model" which is used during validation and in content assist. Builder seems to have performance and memory consumption issues, that we'll take care of in next version.
Validation is incremental AFAIK, but some times it is required to validate not only changed files but other related files. For example changing bean id in faces config xml file should trigger validation for jsp\xhtml files.
Yo can disable validation in Preferences Dialog (Window->Preferences Validation category) for all projects or in properties for particular project.