Same issue here. My biggest frustration is with Gmail notifications being delayed, but I see the same issue with other apps. This appears to be an OS issue where the phone goes into a standby or sleep mode and notifications are not being recognized until I unlock the screen. Within seconds of unlocking the screen, my Gmail notifications pop up. I have my old S6 still active on Wi-Fi in my house and hear notifications immediately, while the S10 will delay notification for 20-30 minutes if at all. This is a widespread issue and I can't understand why Samsung is acting like they've never heard of it. At the very least we need to see acknowledgement and a plan to resolve this. I've changed all settings as recommended by Samsung and others including changing Battery Optimization options, changing Power Mode to High Performance (to minimize phone going into standby), turned off Adaptive Battery option, turning on Always On Display (to minimize phone going into standby), and any other related settings. I really believe there's an underlying issue related to how notifications are being prioritized to wake up the phone. One more thing related to why I believe this is related to the lock screen. I have Smart Lock enabled and use a trusted device (Fitbit) and my trusted location (Home) to keep the screen unlocked, and it rarely works, i.e. I have to use my fingerprint to unlock. I'll admit that the Fitbit can be flakey and doesn't sync regularly as it should ( a different issue that Fitbit can't seem to fix) but the trusted location should always work at my home and connected to wi-fi. This is another data point that the phone seems to go into some standby mode and doesn't respond properly to smart lock or notification settings.
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