Missing or delayed whatsapp messages and calls

I've been having a bit of a problem with Whatsapp messages not getting to my phone when it sleeps and nobody (including Whatsapp and Samsung) has been able to solve it.

However, I believe I may have sorted out the problem.

I noticed the battery optimisation settings are there to make sure your battery lasts as long as possible, but it appears to shut down all apps that are battery optimised. Most apps simply don't matter so I left them alone, but I turned off the optimisation for gmail and Whatsapp - no more problems as all messages come through in a normal time.
I've spent a couple of days testing this by letting the phone sleep, then sending messages at various times after the screen goes out.
What failed before no longer does.