DCC isn't the best protocol for file sharing anyway, and DCC relies on the slowest connection in the transfer - so even if you're on cable and the other person is on dial-up, you're not going to get fast speeds.
There are a few commands you can use, generally they won't help significantly though, or even at all:
/fsend on
/pdcc on
/dcc packetsize 8192
Are you also aware that mIRC is not a file trading program?
Regards,