I have been a service manager at two shops and held a contract for assembly and maintenance contract work at a Sears store when I owned a moble repair service. What makes the largest impact on assembly time and end quality under a given standard is:

• Written standards - Criteria to be met for every assembled bike, with inclusion of every part of the process (as above, but detail for each task)
• Training - not just how to do each task, but how to order and flow them for maximum efficiency. Ideally the best assembler demo's a full assembly, explaining what/why as s/he proceeds.
• Tools and resources - An organized, dedicated assembly station can make a huge difference. I set up an assembly station with a peg board with basic assembly and adjustment tools (or center them if only one board is used for assembly and repairs), a gauge-equipped inflation chuck and pneumatic torque wrench on coiled hoses coming down from the ceiling, and the assembly task list posted in a sheet protector. A dry erase marker allowed the assembler to mark the last step completed in case of interruptions or breaks.
• Monitoring and feedbackj- Slow assembly times need to be approached first by asking the assembler if s/he is having frustrations with a particular aspect of the task. Sometimes that frustration may indicate a need for training, sometimes the need to raise shop standards. If not then monitor an assembly (take notes rather than interrupt) and give feedback about areas for improvement.
  Fast assembly times should be addressed by Q/C checks of bikes and warrantee returns to make sure quality is being maintained, and if so then by finding out what the assembler is doing that may help the shop improve it's processes.

Obviously the bikes at Sears were low-end and relatively simple (1983-1986) but my assembler could complete the entire process in wellunder 30 minutes, and I included proper cable housing sizing and lubrication (unlined). We had about 10% of the maintenance contract warranty work as they had with the previous service, which made both customers and the sales manager happy (and the riders much safer).