Currently, we send 1 message segment per second.
1 message segment is roughly 160 characters.
This means a text message containing 170 characters would equal 2 message segments. Thus, doubling the time to send one full message to a single contact.
To roughly guesstimate how long it would take to send a message take your message segment and input it into time calculator like the Google one below.
If you were sending out a holiday greeting to your 21,500 contacts (through your CRM integration of course) and your message was exactly 160 characters.
That's 21,500 seconds to process your CRM triggered request and that will take 5.9 hours.