I haven't heard anything from Google about changing that interval.
The definitive test would be to go into Google Voice Settings (if you have migrated to the new web GUI, then scroll way down to the lower section of the page, to the "Calls" section), and remove the check marks next to all of your forwarding destinations (10-digit telephone numbers and Chat). Now, your calls won't be forwarded anywhere (well, they'll ring any logged-in Hangouts clients, but for this test, we don't care). In other words, this removes all added delay introduced by the call forwarding from Google Voice's outbound switch, through various transit carriers, to your forwarding phone number.
Call your GV number from a phone not on your list of forwarding phones. Time the ring interval, from the moment you hit the last digit to the moment you hear the GV VM greeting begin. If you are calling from a VoIP service, hit # at the end of the phone number to eliminate the wait time for more digits. Call from either a POTS line or a VoIP land line. Calls from mobile phones are too variable to be meaningful; they may incur some considerable added delay from the wireless phone network.
It is expected that the interval will be slightly longer than 25 seconds, since there will be some telco network switching time included. Total time of 30 seconds would not be surprising. It shouldn't be longer than that.