Instant chat pings the server every 5 seconds. If new messages are available on the server>>chat pulls those messages and shows to the other user.
Other Oxwall plugins ping server as well. For example, update user activity, notifications,invitations, mailbox,friend requests. They have their own ping times. So mailbox might ping every 3 seconds, friend request every 2 second. And as a result you end up with pings happening almost every second.
Two things can cause the delay in message delivery:
1. server's capacity/settings
2. number of users online.
Oxwall can't control both. I guess why we can't control server settings is quite obvious and there is no need for further explanations.
With the second one, Oxwall uses ajax based chat. And ajax technique itself tend to slow down with a lot of users being online at the same time. So we can't control this as well.
>>Is chat linked with a Cron Job??
Message sending/delivery - is not linked to CRON. Whether user is available for chat or not (online/offline) depends on CRON.