Well in my case it was approved in 15 days via premium processing, months before I started working.
But in general you're correct that employees can change work sites while the H1-B petition is pending (because without premium processing that takes months), but only after the new LCA is approved. And the LCA is what determines the prevailing wage for the new site. So you can't move someone from Bumfuck, Nebraska to San Francisco and continue paying Bumfuck wages.
If you think the processing time is too long - I totally agree. 15 days should be regular processing time, not "premium".