When sending a Mail Merge in Outlook you have two options with which to send it. These options are "With Office 365" and "On the Desktop"
When sending granular mail merges with Office 365 there is very little processing time as this is done through the cloud. On the other hand, when sending granular mail merges on the Desktop the processing time can be considerably longer when sending to large distribution lists (e.g. 1000+ recipients). This is due to all the processing being done on through outlook.
When Sending aggregate mail merges with Office 365 and on the Desktop, you will see very little difference between the two processes as processing time for both is almost instant.