So, that points to the Sprint number as the culprit. Typically, the delays in the device ringing are caused by the carrier. Google Voice simultaneously rings all forwarding destinations; there is no setting nor feature to do sequential ringing.
Sprint has apparently been making changes to its WiFi calling and LTE infrastructure recently, related to a new WiFi calling platform and VoLTE. There are a large number of complaints on other forums about Sprint service. In particular, Conditional Call Forwarding, the crucial feature to forward carrier calls back to Google Voice voicemail, is largely broken.
All I can suggest is that you try this:
- From a laptop/desktop computer's web browser, not a phone or tablet, disable and then completely delete your Sprint phone number off of your list of Google Voice forwarding phone numbers. Make sure that there are no other phone numbers listed there, especially numbers that you no longer control (e.g. canceled service, abandoned)
- Now, using your Android phone's Google Voice app, add and verify the Sprint number again. The verification procedure should automatically send a special text message to your phone for verification. You don't need to open that message; the app will receive it instead.
- If your Sprint number supports WiFi calling, turn off that feature, and test as before.
Frankly, now might be the time to finally leave Sprint for a better carrier.
