>>>i don't know how to change the setting of XNOTIFYTIME system registry variable

It is easy enough:
At the command prompt type:
(setenv "XNOTIFYTIME" "1")
and hit Return.
This will set number of minutes between checks about changes to one minute. (By default, the program checks for changed xrefs every five minutes)

I also can suppose if your external references are on the network drive, you can have delays in notifications regardless sys var settings.

Maxim