I whouldn't worry about that.
Although many people think playing 5 chars at once you instantly kill everything and therefore you must level very quick there are different problems to deal with like:

* managing macros
* some char accidently dying
* many aoe spots are cleared very fast but the respawn is not fast enough so you want to go some place else
* Crafting, organizing ressources (so what do i do now with the 436 bolts of linen i have looted :P), feeding the auction house
* etc.

Expect to be slower than a single character but do not worry about it.

Enjoy the game experience - thats the most important thing,
Bunny