Slow Duplication Jobs Which Then Slow Other Jobs Down

Since you are on 7.6 I would implement SLP windows so duplication occurs during day time. I would also ensure backup has higher priority than duplications. Doing writing and reading on the same disk will impact overall performance so best to separate the two work loads.

LIFECYCLE_PARAMETERS is  deprecated in 7.6. The same setting can now be found under the master server settings. But you won't find a setting that solve youre issue, however tewaking workload may ease the issue. 

There is a TN describing the meaning of the SLP setting, it written for 7.5 that still use the LIFECYCLE_PARAMETERS file. Just ignore that.

http://www.veritas.com/docs/000023582

Doing duplication from disk to tape is per definition non-multiplexed.Preserve multiplexing is for duplicating tapes only.