Custom File Execution Analogy

Hello Team, I have designed a chatbot using RASA (nlu and core).
Problem I am facing :-
Suppose there are 3 users - u1,u2,u3.
All these users clicked on a functionality “Get Restaurant”. at -
u1 at t= 0
u2 at t = 20 sec
u3 at t= 40 sec
and suppose “Get Restaurant” functionality takes a long time : say 2 minutes.\

action_get_restaurant , a custom action will run for “Get Restaurant”.
My Query is - Will all the 5 users will get ans like this -
u1 at t = 0+120 = 120 sec.
u2 at t = 20 + 120 = 140 sec.
u3 at t= 40+120 = 160 sec.
or, first u1 will be completed then u2 then u3.\

I know my query might sound idiotic but i got sequential results and not parallel.
Actually I have one big functionality which takes 2 min to get results . if a user had clicked on that big functionality , then other requests seem that they r in queue. As that big functionality return results all the other queries get executed.

Kindly tell me why i am getting such scenario. Secondly kindly tell me Analogy of RASA core, it supports parallel processing of queries or sequential

Kindly help me to understand it .