The London office processes applications from most of Europe (except the ex-USSR), plus skilled migrants from the Middle East, Africa and North America. That means working with documentation in over 30 languages--even with translations provided, the verification process can be time consuming. Shanghai serves China and Mongolia: two languages (in addition to English); Beijing only India, Pakistan, Bangladesh, Nepal and Bhutan - perhaps a dozen languages.
Applicants with job offers are processed more quickly--very quickly, in fact. Mine was processed in 2 months. It would be great if they could be done more quickly; it's important they're done thoroughly and fairly.