Is there any difference between poczekać and zaczekać?
There’s not much of a difference between the two. Both are used in the same contexts and are equally common.
Some people might tell you “poczekaj” sounds more impatient than “zaczekaj”, but most speakers don’t pay any attention to that and just use them interchangeably.
“Czekaj” (without a prefix) sounds definitely more impatient/informal though.