Required Client Deployments Never Kickoff On Time 2012 SP1

SCCM engineers,

With other OSD tools, when you design a deployment to kick off at 8:00pm, you actually get that (or pretty close to it).  With SCCM 2012 SP1, we've never had luck getting required deployments to start on time...which is a problem when you have schedules and deadlines.

1. Anybody had any issues with this before in a mid-sized organization (~20k workstations)?
2. Are there other posts/resources out there I could take notes from?
3. What is the most realistic expectation from the tool in terms of meeting an assigned start time?

I'd be happy to give any specifics about the configuration if necessary.

Thanks!