event:15 does but
eventtime:15 doesn't as far as I can see?
SQL is not faster because it's SQL, far from it. SQL can suck hard from a performance point of view.
A large part of my worries stem from the fact that I'm not that familiar with the strengths and weaknesses of MongoDB and I am really familiar with the challenges that MySQL can pose when it comes to forums and big boards.
In the MySQL case the DB can be really slow if proper indices are missing. And that's with database tables which are highly structured where it's trivial for the DB to do a table scan on a certain field.
I view MongoDB as one large bucket of JSON objects that are hardly related or structured. It's possible to index them to some extent but as far as I know that's not really efficient when everything is part of one large collection.
But most I guess most of it is 'fear of the unknown' I guess I should just fire up a conversion or fixtures script and see what happens...