Slow processing of messages in the Inbound folder can be caused by a couple reasons.

Rules Processing:
The parsing of rules can be very processor intense, and with large emails, may take longer than expected if several rules are stacked.  If you are going to have more than 20 rules(including domain, user, and system levels), it is best to add the rule action to "terminate all other rules".  This will help optimize rules processing.  SMTPMax will run at or close to 100% till rules are done being processed.

User disk limits:
If a user has a disk limit set, MailMax has to iterate through all current messages in the folder to determine if the next message can be delivered.  This can increase the amount of time to process the message if a user never checks their email, and a disk limit of 5MB or more is set, or if tons of messages reside in the users folder.

Another thing you may want to try is opening the users folder in Explorer.  This can be a good gauge to how long it should take MailMax to write to the directory.