Tandems (especially those built for road racing) are superb on the flats and downhill. Where a single might average 20 mph all day the tandem will probably average close to 25mph.
Uphill is tougher on a tandem than on a single. When riding with singles, you'll see them getting spit out of the pack on a long uphill.
When I was riding with a club, I had a tandem couple I always used to draft. On uphills I'd move along side and gently push the stoker to raise their speed up the hill. On the down hill I'd tuck and stay in their wake. On longer/steeper decents, I'd begin to move faster than the tandem
at which time I'd again push the tandem and again tuck back in behind. One such occassion was our only sub 4.5 hr century (100 miles)