Exchange 2010 - shared calendars slow syncing

Dear,

Branch office users are running outlook 2010 (in cached mode).

exchange 2010 server is in the main office (connected by 2MB fiber)

users shared theirs calendars (some calendars are shared with 10 users)

when an appointment is set in a calendar, it will take a long time (sometimes 1 or 2 hours) to sync this element with the outlook of the others users (sometime just a few minutes).

is there a way to improve this ?  (we want to stay in cached mode)

thanks