Charles,
there's a huge variety of possible reasons for poor performance,
unfortunately.

You can try a few things to encircle the problem closer:
Be sure you don't use the option "quiet" in dsm.opt
Now you can see which file gets backed up at which time. Do you have
poor performance from start to end or is there a huge gap of inactivity?
Is your network interface configured as it should? Check for mixed
fullduplex/halfduplex environments. We had cases where it worked fine
with numerous machines and one linux client sent 40kb/s instead of 7mb/s
until we found the misconfigured NIC.
extend the summary information your client creates after session
completion.
Put the option "traceflags instr_client_detail" into your option file.
In addition to the summary information you usually get after backup
you'll find something like this:

------------------------------------------------------------------
Final Detailed Instrumentation statistics
Final Detailed Instrumentation statistics
Elapsed time:  1502.420 sec
Section      Total Time(sec)  Average Time(msec)  Frequency used
------------------------------------------------------------------
Client Setup       15.081        15081.0              1
Client Setup       15.081        15081.0              1
Process Dirs      331.908          191.4           1734
Solve Tree          0.000            0.0              0
Compute             1.021            0.0          76446
Transaction        43.535            0.2         244347
BeginTxn Verb       0.000            0.0            269
File I/O          733.274            8.2          89169
Compression         0.000            0.0              0
Encryption          0.000            0.0              0
Delta               0.000            0.0              0
Data Verb         474.198            6.2          76446
Confirm Verb        0.251           16.7             15
EndTxn Verb       195.729          727.6            269
Client Cleanup      1.612         1612.0              1
------------------------------------------------------------------