I am facing this problem with some of my users they complaining that they receiving account activation email after 5 to 10mins however for some users it is instant.
Please let me know is there anything I can do to make this instant for all the users?
Email is by design not instantaneous. There are many reasons why an email may be delayed, ranging from overloaded servers to greylisting. Unless your Discoures instance is very busy, however, it is extremely unlikely that Discourse is delaying emails.
There is no delay on the Discourse side for activation emails. They are sent to the SMTP server as soon as the user signs up.
There is a default delay on notification of replies to give people time to edit the reply in the edit grace period.