generally, a good paddler in a solo canoe can keep up with good paddlers in a tandem if you keep to a reasonable pace (i.e not flat out as fast as you can go) - a not so good paddler will have more trouble keeping up with a tandem team, even if they are also not so good - its just easier in a tandem

the difference isn't so much in speed as in the amount of work it takes to keep up that pace.  The solo paddler is going to be working harder, especially in headwinds where you can't just coast for a bit to take a break.  Consider also that that difference is cumulative - you'd be wanting to switch off in the solo.

I bought my first solo boat for a 3 person trip and never looked back.