Send Speed

I have no idea why this happens. You can enable the debug mode and check the SQL queries. Maybe this will help to find the query that requires so much time.

Saving a message only stores a few information which does not require much time (one query that inserts the message itself and - if it contains an attachment - one query that inserts the pointers to the file attachment).

How many messages are stored in your database? Maybe your server has not enough ram to hold all index files in memory?

Here are the stats from the back-end:
Messages stored in database: 20940
Messages trashed by recipient: 10079
Messages trashed by sender: 436
Messages on hold for purging: 7
Messages: 23589
Messages in 7 days: 43
Messages in 30 days: 482
Messages in 365 days: 16805
File references in database: 4962
File attachments stored: 1622

Excessive?

No seems to be ok. I did some tests with lots of more messages and file attachments. Have you checked the queries (debug mode) that are done to store a message? Maybe this helps to identify the problem.

Still working on this issue and it is still a problem, just to recap:
1. If we send a message with the "attachments" feature enabled, even with no attachment in the message, the Send process takes 15 - 20sec. When "Attachments" are disabled, the time is <2sec.
2. We have checked for queries, none are running during this period.
3. If we double the cpu and ram on the hosting server, the time is reduced by roughly 50%, but still not ideal (and very expensive). So the "attachments" process seems to be very resource intensive, even when there is no attachment.

I think our users wouldn't mind the wait if they were actually sending an attachment. Is it possible to have uddeIM check for attachments first before running whatever processes are consuming such high resources? That way messages with attachments would be slow, but normal ones would not.

I have checked the code again and again and there is no reason why it should take longer then normal. When no attachment is added uddeIM does not execute any time consuming commands, not it does any additional SQL queries.
Maybe it comes from a another component that tries to process the empty $_FILES array, don't know. I cannot see a problem with uddeIM attachments.

I agree that the problem may be related to the empty array, it may aggravate I/O bottlenecks when storage is on separate hardware. I'm not exactly sure, but this may be my point exactly. Is it necessary to create that array when a message contains no attachment? In other words, wouldn't it be more efficient to have the component behave like it does without the "attachment" feature enabled when there is no attachment?