It is needed for anti-spam reason.

Many systems refuse your connections if you don't have a valid PTR record (and if your helo don't match it...)

What Gabriele said here is correct; the reverse DNS is a standard antispam check that is performed by most if not all email providers/servers. 

The long delays you are seeing when performing RDNS lookups could be stemming from DNS delays, so I recommend retesting those scenarios with some common public DNS servers in place temporarily to see if the issue improves. You can try the following which are known to be a good starting point - and these are configured in Settings>General in SmarterMail when logged on as a system administrator: 

Maybe I had been misunderstood here. IP which has no RevDNS is my clients IP from ISP which connects to my SmarterMail main instance which is not performing any antispam actions (I do that on incoming gateways). Just to clarify it here: this IP is not IP of any mail server or any mail gateway but just client IP from his ISP.

Issue here is that when client is sending email from his Outlook sending message window is stuck for one minute because of SMTP session being hang because o RevDNS checking on SmarterMail side.

I've used tools like (just to be sure):

to verify RevDNS and users IP addres does not have one so changing DNS servers will not change anything here.

My point here is that if I don't perform any antispam operations on my main SmarterMail instance agains incoming SMTP connections (especially ones which are authenticated) does SmarterMail must perform RevDNS resolving in first place?

I would like to return to this topic with a question:

If no spam checks are being done on SmarterMail instance to which SMTP client is connecting then is there a need to perform RevDNS resolving during incoming SMTP session?