On a ride with an average amount of hills I am a little slower on the tandem, due to the stoker being slower on a solo bike than I would be. The tandem is probably around 15% slower up the hills, same on the flat, and a bit faster down compared to me being solo. On group rides/races it is also slower as the bunch you are in is mainly determined by how fast you can climb.
Regardless of the speed though I would still rather ride the tandem. It is great working together and achieving things as a team.