Kaiser,
One more bit of info that might help. The SM200A in fast sweep mode steps in 160MHz steps. If the span you select happens to be in two 160MHz steps, it will have to step the LO twice for your one sweep. If you shifted your 100MHz span higher or lower you will find a spot where we only have to step the LO once for your sweep, you could get a theoretical 2x speedup (minus some overhead). If you have the flexibility to move the span for certain measurements, you could utilize this.
Regards