I've read that Vietnam-era US fire support typically took up to five minutes to arrive whereas NZ and Australian fire support in Vietnam (using British practice)could take less than 90 seconds.
Reasons given were;
British-style guns were calibrated directly for range as opposed to elevation.
British-style observers could order gunfire as opposed to requesting same (see the point above about unneccessary obstruction)
Obviously a lot of other factors came into play for both pratices(competence/training, air clearances, pre-registration, etc)
How much of this is over-simplification others can say but a place to start is;