Since this is the top result in google searching for gmail delayed notifications, I found this to fix the problem on my S9 Galaxy. I didn't come up with it, found it on a different forum. This does not require the phone to be rooted. Basically this disables some form of deep sleep in the phone. I haven't found it affected battery life much, if at all, but I am sure that's why this feature exists. Also, this will only disable the deep sleep feature until the phone reboots. At that time it will return or normal and you would have to re-run these commands. I don't reboot often so it isn't a huge deal for me. Also I am running the commands from a windows PC, but any ADB client can do it. 1. Download and install the Samsung USB Driver onto your Windows PC. 2. Download and install the Windows ADB (Android Device Bridge) software from Google (just need to unzip it, no actual install procedure). 3. On your Samsung phone, go into Settings and check "Enable USB Debugging" 4. Connect your Samsung phone to the Windows PC and it should pop up on the phone and ask to allow USB debugging, you want to allow it. 5. Open a windows command prompt, navigate to the folder where adb.exe exists (where you unzipped it). Run the following commands (two separate commands): adb devices <-- should show one entry under "list of devices attached". If it says "unauthorized" then you need to make sure and allow USB debugging. You may need to go on the phone under settings and "Revoke USB debugging authorizations" and re-allow it. adb shell dumpsys deviceidle disable <-- this will disable light and deep idle 6. That's it, unplug the phone from the PC and you're good to go. At least until you reboot.
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