We had the same experience with Solaris (though I can't confirm it was
2.6).. Same exact symptoms.. FTP was lightning fast but TSM was a slug..
cpu, disks, and memory (1GB both sides) on both client and server were
relatively inactive during the backups. Both Sun boxes were new. Tried all
kinds of TCP/IP tweaks but nothing helps *at all*. In the interest of time
we gave up and threw TSM on an AIX server instead. The network guys saw long
pauses between acknowledgement packets but nobody could figure out why.

Unfortunately I don't have any real input but just wanted to chime in that
I've seen the same problem with a 4.1.2 installation.