reduced basis set

When starting work on a large unit cell it is wise to start with a DZ basis. With such a basis, any of the above suggestions can be tested. However, for most properties, the DZ basis is probably not very accurate. You can next go for the TZ2P basis set, but that may be a bit of overkill. One reasonable trick is to remove f polarization functions (if present) from this basis set. So for copper you can comment out the 4f STO

AtomType Cu
DIRAC Cu
 7  5
VALENCE
1S
2S
2P
3S
3P
4S  1
3D
SubEnd
BasisFunctions
  3D     1.280000
  3D     6.900000
  4S     0.850000
  4S     2.450000
  4P     1.370000
!  4F     2.200000
SubEnd
FitFunctions
  1S    48.900000
  ...
  

Note that this will save you 7 functions per atom. If this is still unwieldy, you can also decide to start from the DZ basis and add a polarization function by hand.