You can already do that. Hit Alt+O to open mIRC Options, go to DCC > Fserve. Change the "Total Max. Cps" number to anything you want (cps = bytes/sec, so if you want 10 k/s, enter 10240). Then start all your sends with
/dcc send -l nickname filename
/dcc send nickname filename
Even though the setting is in fileserver options, it applies to normal dcc sends too.