You can only go as fast as your connection can, but there are a few DCC options you can set to try to get the most out of a download.
/help /pdcc
/help /fsend
And you can also change the packetsize for a DCC using:
/dcc packetsize #
Where # is a packetsize from 512 to 8192 (bytes)
Note: Most of these (I think all except packetsize maybe) are for speeding up sends, but who knows
