NOT A TEACHER
I have found a "split-second decision" here, so I think that it's fine.
Student or Learner
Could I say "It's been/was a split-second decision" to mean that I didn't have time to think as if I were trying to justify myself?
"It was" is fine. "It's been" does not work.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.