How many KDF rounds for an SSH key?
I did also tried to find a good value for the -a flag, in a MacBook Pro Mid14 (i7), trying to login in to a Debian 8.5, I had this results:


-a 1000 Took about ~20s.
-a 100 Varies between ~3.5s to ~6s.
-a 64 (4x default value) ~3 to ~9s.
-a 16 (default) ~2s ~2.5s.


In the end I just stick with the default since I'm not a paranoiac and I don't like to wait, however I did found in my research a value of 100 is not uncommon to choose.