That may be the case, but it's definitely not a universal problem among all Android phones. I've never had issues with delayed notifications on any of my Pixels (at least, as far as I'm aware). I have seen this issue raised with Samsung phones, so it could have more to do with how a manufacturer tweaks the firmware. Certain manufacturers are known for overly aggressive RAM management in a misguided attempt at improving battery life, as described here:
https://dontkillmyapp.com/