Worst case execution times are generally unknowable, even at codegen. It depends on things like the specific microarchitectural details, what other instructions are currently executing, the memory hierarchy, what instructions have been executed before, how the power supply is responding, what the thermal environment is like, etc.

HAL/S dealt with these problems by a combination of programmer/validation discipline, custom hardware designed for hard real-time usecases, and generous support from the underlying RTOS (FCOS). Cryptography code doesn't deal with it, it just avoids data dependent nondeterminism, which is good enough for those use cases.