Answer
The setting is found in the PXE.ini file, in the line labeled "BandwidthThrottleInKB=-1", and in the PXEManager.ini file, in the line labeled "BandwidthThrottleInKB=1000". Both files are found in ...Program Files\Altiris\eXpress\Deployment Server\PXE.
Note: The PXE.ini file can be automatically updated or replaced during PXE services restart operations or PXE information updates, but the settings are duplicated from the information in the PXEManager.ini file.