We did 15th march.
Southampton bag drop was operating at 9:55.(don't know how early they started).
While I went to park(cheap off site) OH said security opened around 10am.
By the time(10:12) I got back no line, anyone that had been waiting had been processed and on ship.
8mins to do security, check-in and walk into ship.
We were in the buffet for breakfast 10:23
A few on the other place group reported being on 10-15 mins before.
NOTE: MSC summer cruises do get a lot busier as they can be full ship embarkation.