ASLR calculates 3 random values on execution and saves these as offsets
   to be used when a random value is needed. How expensive are 3
   arc4random() calls in the context of an entire sys_execve()?


you fail to understand the performance issue here.  when, eg, libc is
not mapped at the same address as other processes, the performance hit
is in the range of 30-40% on some platforms.  it's not about start up
it is about the MMU being constantly trashed.