Including additional authors to reduce time I tried to find the same question and failed, so here it goes.

Imagine the following scenario: you have invested time (say 8 months) ona  research project. You would probably get a publication in 4 more months of full time work. However, if you include two experts (who provide code) you could reduce the additional time to roughly one month. In both cases you would remain a first name author.

The trade-off is between the number of authors of the paper and the time you invest.

Would there be a rule of thumb to help me with this decision? Is there a clear answer, or is it completely subjective?
As some comments above indicate, there is an important rule of thumb from the field of software engineering, which is Brooks' Law.

According to Frederick Brooks, in his book The Mythical Man-Month, adding more workers to a project which is already behind schedule, is likely to make it even farther behind schedule as new members are brought on board.

Take the time to consider whether you can beat this effect before attempting to grow your group.