You are running DLP Endpoint Prevent and using the Outlook add-in, and users report that some messages take a long time to send, for messages sent to a large number of recipients (at least several hundred).
The delay occurs due to the need to enumerate each recipient against the DLP policy set. This is expected behavior.
To avoid this issue, create a distribution list if users need to send messages to a group of recipients frequently. Also, you should instruct Outlook users not to "expand" the distribution list to show each recipient individually, because this will cause the same issue.