Transfers from dynadot to other registrars also happens the same day, Dynadot sends a final authorization request to domain owner once the current domain owner authorizes the transfer , as per as my experience with in 2 hours the complete transfer can be completed. I think its up to the loosing registrar in implementing their own process in loosing a domain from their account.
We are also adopting same five days approach but in some cases where our clients request with reason then we also releases the domain name.
Why five days ?
If any fraudulent transfer takes place then this enough time to alert the registrant where they have setted some kind of whois change alert services.(Alert services provided by lot of third party sites but not by Mitsu).
Five days period is in the interest of customer only. Transfer is not the services which is used by customer frequently , in a domains life a regular customer takes transfer three to four times max.