many (75) port 445 connections = slow

Is there some way to improve network performance when one has many (~75)
port 445 connections open.
I have a sever that does nothing but move files. as the number of
connections increases we see longer delay in accessing new connections or
even accessing the box with remote desktop.
the connection are to servers over VPNs, some are slow dsl or cable lines,
some are t1 or DS3 lines.
Not sure where to start looking to get things tuned.