If you can afford the time, effort, and expense needed to move to China, maybe you should look into working with a local board shop that could provide more rapid turnaround.
The best way to improve your development time is by doing fewer iterations. Often easier said than done, but you should be designing for test and rework and thoroughly reviewing your designs before submission. You said you're making flex PCBs -- have you considered doing fast-turnaround prototypes on rigid PCB to make sure your electrical design is good? What kind of problems are you having that are forcing you to do extra prototyping runs?