Gadgetbridge has very limited aGPS support, right? Without aGPS updates, any GPS device is going to have terribly long lock times.

Ideally, watches should do like Garmin's. Mount as mass storage devices via USB, and let the user download activity data and upload updates or routes.

For my Instinct 3 Solar GadgetBridge it - as far as I can see - can deliver AGPS files.

Settings -> Location -> AGPS