finish-heavy-allocation tells the system that the application finished doing 'heavy' allocation, and from that point onwards allocation is 'normal'. The main distinction between heavy and normal allocation is the typical lifetime of objects: normal allocation means most of new objects are ephemeral, while heavy allocation a large proportion of the new objects are long-lived.
Heavy allocation normally happens when loading, either the application itself or large amount of data. Operations that do not involve loading will almost always be normal. Hence the time that is useful to call
finish-heavy-allocation is after loading something.
LispWorks User Guide and Reference Manual - 13 Feb 2015