What you are seeing is an SMTP acknowledgment delay on office365 which cannot be changed by anyone other than M$ (not that I have found to date anyway),
on an internal Exchange you can use the following powershell commands to turn off that delay and Tarpitting which
can also effect the speed of SMTP sends;
Set-ReceiveConnector "connector-name" -tarpitinterval 00:00
Set-ReceiveConnector "connector-name" -MaxAcknowledgementDelay 00:00:00