If you are compatible and not riding in a pack, very hilly country, cross-winds or around town with lots of starts and stops, I think you will be faster to a degree you will definitely notice. You will have to try to find out! I don't just base this on the physics which favor tandems in a lot of situations, but also on the additional communication you get through the timing chain which can really give you extra motivation to go hard. There is no equivalent on a single.

So why are you interested in riding tandem, if you already get to ride together on singles? Since you mention speed, I devine some competitive impulse here.

Do you want to smoke the pack you usually just hang with? The tandem may help, but they will make you suffer on the uphills. If you just want to hang with the pack, it will be harder on the tandem due to momentum differences.

Do you want a PR for a rally or timetrial? The tandem can really help if you don't feel like it's cheating.

All your friends have tandems and you want to join the fun? Go ahead! Someone will probably lend you a bike for a ride or two.

Enjoy being different and having people point as you ride past? A tandem is in your future......