So you add atoms to a lattice to improve performance, but why does filling that electron spot improve performance? Does it fill the gap?

Well, here by ``performance'' I meant ``conductivity''... when you add dopant atoms, you add extra electrons or holes at some sites. Yes, doing this does provide extra (localized) states that electrons can occupy in the gap between the valence and conduction bands (although you wouldn't say the gap is filled).

For a donor impurity (making an n-type semiconductor), electrons in the new states can easily jump to the conduction band. For an acceptor impurity (making a p-type semiconductor), electrons in the valence band can jump to the new state, leaving a hole that's free to travel.

See the diagrams in Eggleston 3.1.3.