Benchmark it once on each device. Then have a user-friendly slider.
"Do you want your security to be:"
a) "It only secures pr0n from my aunt" (1s for fetching a password)
b) "Not great, not terrible": (5s for fetching a password)
c) "Pretty Good Protectivity": (10s for fetching a password)
d) "The CIA haunts me and my name is Edward: (24 minutes for fetching a password)
"Do you want your security to be:"
a) "It only secures pr0n from my aunt" (1s for fetching a password) b) "Not great, not terrible": (5s for fetching a password) c) "Pretty Good Protectivity": (10s for fetching a password) d) "The CIA haunts me and my name is Edward: (24 minutes for fetching a password)