I noticed this a little while ago, and @Morgan_Hardwood recently brought it up again. We seem to be experiencing some slower-than-usual uploads into posts. I am trying to bribe @darix into looking into this with me, and hopefully we can get to the bottom of it.
It may be a hardware issue, in which case I'll bump my timeframe for migrating everything to bigger/badder hardware.
To illustrate, here's uploading a file onto the meta.discourse.org instance:
And here is uploading the same file here on discuss: