Hi Hari,
Ok. Looks like the job setup is fine. I am not expecting Job allocation to be blank though but probably not the root cause. I am assuming that you have read the related OSS notes and still no clue on this. Two approachesI would take. My preference is actually technical one as that is the most reliable. Nevertheless, you decide.
The technical approach will be do a performance trace using just a selected number of billing docs in background. That should give you clue on what is the abap program doing. Where is the specific bottleneck?
A more functional approach will be like peeling the onion. I would get my SD consultant to reduce the processing as much as possible. E.g. Disable output from issuing. Disable LIS, Disable Customer Modifications etc. Basically strip down the potential processing in update task and isolate the feature that is the root cause. There are situations that you can delay certain processing. You can populate SIS, issue output, trigger idoc in separately with background job that aren't necessary to be synchronously during Billing Creation.
Anyway, please do not hesitate to reply to this thread if you have further questions. Hope the above helps. I would partner with SD and Dev.