mortgaging
once you have a monopoly, it may be time to mortgage.
As indicated, development speed can be critical, thus once a monopoly is acquired, all efforts should be made to acquire funds for building. This would include selling any properties that are not likely to become monopolies (with consideration given to avoiding creating competing monopolies, or at least, leaving such a competitor with little in development funds), and mortgaging whatever properties are not sold. An exception might be railroads that are part of a group of three or four.
Unmortgaging properties should not occur until, at least, all properties that can be developed to the three house level have, in fact, been developed.
