Got 'Speed Reader' after an hour...

As I was progressing through the campaign on Legendary, solo, I was just taking my time going through the Library and just taking in all the visuals.

After an hour or so I finished the level, and to my surprise I got the ‘Speed Reader’ Achievement.

Is there a reason for this? Is it known to be buggy? (Not that I’m complaining…)

someone posted a comment on that before.

time resets to the time of the last checkpoint if you die…or something like that