It is currently guarantee(), which slows down release bits unnecessarily:

inline int ZeroStack::abi_stack_available(Thread *thread) const {
  guarantee(Thread::current() == thread, "should run in the same thread");

On my TR 3970X, changing that `guarantee` to `assert` improves:
 - Zero x86_64 release "make images" times from ~9.5 minutes to ~7.5 minutes
 - Zero x86_64 release "make bootcycle-images" times from ~49 minutes to ~44 minutes