BR runs longer in a year with 3 months than in a year with 12 months of data

Summary:

We've got a forecast in FY23 with 12 months of data and 3 months of data in FY24. The data has the same detail, but FY24 runs twice as long. (Edit: Calc manager log also shows more block reads/writes etc. for FY23, but overall lower runtime).

Content (required):

As described above. I have already cleared empty blocks/run a restructure, but to no avail. The BR hasn't been changed and runs as expected in other Years/Scenarios/Versions etc. I have also run a data extract for both Years and FY24 does indeed contain only a third of the data compared to FY23.