After a lot of analysis with a user, I found the reason.
The Motorola Home is blocking all widgets refresh when screen is off (phone standby)…
and the problem is : it order the refresh of ALL widgets on the Home screen when waking up !
Result : On last logs the user sent me, it take 6 seconds to refresh 6 widgets !!!
My advise to avoid this: drop this Home and use a good product like ADW or Go Launcher.