Welcome to mIRC
Just so you know, mIRC is a text chatting
piece of software. If you're using mIRC as a file sharing device to just find files, you're in the wrong area. mIRC is probably one of the least effective methods of transferring files.
At any rate, You are always limited in 'speed' by the slower connection. You could be on a T3, but you're only going to be able to send files as fast as the person on a 56k dial up can get them. You're also limited further if the other user is getting other files.