if you run your own inbox relay locally, you can get notifications in literally the speed of a network packet from the sender (plus maybe a few microseconds for processing) with this, with anything else it has to go through a notification service, which means at best you get 2 packets speed (and usually more). 

If you don't run it locally, it's the time it takes the sender to send a network packet to your inbox relay (plus a few ms) + your ping to your inbox relay. 

Literally nothing can beat this, assuming perfect code naturally