From our point of view, shortening a single activity usually means one of 2 things;

Increase resource availability
Introduce another shift

Both obviously have their own constraints and effects on efficiency

We do basically what your guide says, starting with our critical paths and double shifting work or increasing team sizes to shorten them. We do this manually, considering the cost of each change as we make it.

We’re not trying to save indirect costs as far as staffing’s concerned but lost opportunities in unit production, which often exceeds the work costs themselves. Sometimes we are constrained by the total labour force availability, thus we can only shorten the critical paths by a certain amount before we generate Critical Mass in the multitude of long lag activities. This is less common in newbuild projects where most activities rely on others therefopre creating many long paths. Most of our scope items are independant of one another.

We are also keenly aware that generating multiple critical paths is to be done with caution.