The first 2 loops are redundant:
Reduce the reading/writing operations as much as possible.

sub M_snb()
   sn =sheets("BSA|AML").Cells(14 ,6).resize(126,12)
   redim sp(1 to ubound(sn)*ubound(sn,2),0)

   for j=1 to ubound(sp)
     sp(j,0)=sn( (j -1) \ ubound(sn,2) +1,(j-1) mod ubound(sn,2)+1)
   next

   sheets("Import_Template").Cells(2, 8).resize(ubound(sp))=sp
End sub