Good Morning Hopegirl,
I will try to answer your question but please remember I am not a professional.
Xanax: Peak plasma concentration is achieved in 1-2 hours. It is the drug with the faster onset of these 2 meds you are asking about.
However it has a half life of 6-12 hours.
Half-life: time taken for blood concentration to fall to half its peak value after a single dose. . This time may vary considerably between individuals.
Ativan: Peak plasma concentration peaks at around 2 hours.
The half life for Ativan is 10-20 hours so it is the longer acting. Reference: PDR
You are right in your statement. Take care and keep posting.