network browsing, netbios overhead, poor TCP windowing and a whole slew of other stuff make it slow.
BUT, it should only be marginally 10-15% slower than FTP at most.
Load netbeui on both machines, copy a single file from one to the other and time it. divide file size in KBytes by time in sec to get a guestimate rate in KB/s.
Do same file with FTP and compare. Nebeui should be faster than netbios/tcp.