Program is being built in three phases: get the language working, pin its shape down exactly, then make it fast. Each phase only begins once the one before it is settled, so the ground never shifts under work that depends on it.
We are on Phase One . Its goal, a fully working language specification, is in progress . Phase Two and Phase Three have not started yet.
A working language specification: reversible event handling, effects, sandboxing, drivers, a first serialised syntax tree and rendering into an initial set of languages.
Build context into the language and make mode-based programming possible, pinning down exactly the format of the language.
Low-level performance and highly platform-specific code generation, once the exact format of the language is settled.
Performance work and platform-specific code generation only pay off against a format that will not change, so they wait for Phase Three. Pinning the format down, in turn, depends on first having context and mode-based programming in the language, the job of Phase Two. Phase One proves the whole idea works end to end before any of that is fixed in place.