The Antlr4 framework provides both an SLL (simple) and LL (powerful) mode. To support all Groovy syntax LL is needed. However, for users needing a subset of Groovy, e.g. perhaps a DSL scenario, SLL could be sufficient and may be significantly faster. This issue is about allowing users to enable trying the fast mode (with a fallback to the normal mode). It will be disabled by default. This feature is initially incubating.