The compute time for BSE and Cassida type calculations grows with the third power of the number of included occupied and unoccupied bands
VASP always uses the orbitals closest to the Fermi-level, and NBANDSO ( ) and NBANDSV ( ) determines how many occupied and unoccupied orbitals are included. The defaults are fairly "conservative" and equal the total number of electrons/2 (this usually implies that all occupied state are included). For highly accurate results, NBANDSV often needs to be increased, whereas for large systems one is often forced to reduce both values to much smaller numbers. Sometimes qualitative results for band like Wannier-Mott excitons can be obtained even with a single conduction and valence band.