I also managed to return performance to pre-infix levels by the simple expedient of turning on compiler optimizations; on my machine it yields a 2x speedup. Startup time is now 2.6s, and tests take 35s to run.

Finally, the interpreter no longer dies when it encounters unbound vars, or when trying to invoke a non-function.

Update 9 hours later: After a little more optimization, wart will now start up in less than a second if the binary is up to date. The first run after downloading (build+run time) takes 20s. Tests run in 10s (excluding build time).

I tried to be clever and assume frequent building if running tests, but it turns out wart now does enough work that optimizations always improve build+tests time.