Is there an expression for the following:
A: Let's go for lunch.
A: I am coming in a few minutes.
B: Okay, but be a little bit faster
Do native speakers say this? Or is there a better expression or words construction for this?
"Hurry up" is the way this sentiment is most commonly expressed.
The whole conversation is a bit odd to me.
First example: If A suggests going for lunch but won't be ready for a few minutes, then he or she is likely to say that upfront: Let's go for lunch in a few minutes.
Second example: If you use the present continuous for the future (I'm coming in a few minutes), you imply that you have already made plans or arrangements, which does not fit with the Let's go for lunch.
Third example: In a few minutes already seems quite fast to me. A shorter time than a few minutes is one minute or a few seconds, so why not something like: (Why) can't we go now?