Take a large file (at least 100 MB, preferably larger) and FTP it from one
of your servers to your OS/390 host.
See how long it takes.
Do it 3 or 4 times until you can get consistent timings.

Then try backing up that large file with TSM 3 or 4 times, see how long it
takes.
(Preferably use the TSM command line, and turn compression off to make it
comparable to FTP.)
.
TSM will never be quite as fast as FTP, but shouldn't be much slower for  a
single large file.

If they run at (about) the same speed, it's a network or host problem, not a
TSM problem.
Let us know what you find out.