You’ll feel a little dizzy, pass out after about 2 1/2 minutes, cardiac arrest 2-3 minutes afterwards. About as painless as you can get.
Actually, not even that. There have been numerous incidents where people have entered nitrogen filled tanks and collapsed immediately.
Then someone tries to rescue them and they collapse on top of the first victim.
And another person trying to save them collapses on both of them.
When you enter a pure nitrogen environment, you don’t even get dizzy or have time to understand what is going on. You go right out.