I primarily use a nearly-bottom end android phone that's a few years old and just recently switched to an even older, even lower end android phone that is six years old. Neither has that issue.
Obviously, I'm not really claiming that it's not possible people are experiencing this issue, but it can't possibly be widespread.
I feel like most likely people are using android skins that aggressively kill apps in the background.
I have that exact issue on a couple of not exactly low end Samsung phones. Holding them side by side with signal open. Delivery times vary wildly. Whereas WhatsApp just works (though I hate it for other reasons)