I’ve been using Signal (the apk downloaded from the site, not the play store one) for a long time, and I’ve never faced a delay when sending or receiving messages until the last few days. My dad started complaining that he hasn’t received any of my texts, and he is not able to send images to me either.
In this case, he just had to update the app.
But people face this issue when their phone itself decide to doze off Signal. If you are facing the issue, you might as well disablle battery optimizations, install APK directly (or build one yourself), or change your ROM.
https://stallmansupport.org/debunking-false-accusations-against-richard-stallman.html