I understand that your Apple Watch is taking a very long time to start up each time you turn on your watch. The only remedy you've found is to force it to restart, but the problem still persists when you power on your device normally the next time. I'd like to help you find a permanent solution for this issue.
I recommend that you unpair your Apple Watch and iPhone as a next step. This process will create a backup of your Apple Watch before erasing your device and allowing you to pair it with your iPhone once more. Follow the steps below to complete this process:
Unpair your Apple Watch and iPhone - Apple Support
Once you're done, use these steps to set up your Apple Watch.
Set up your Apple Watch - Apple Support
Try restoring from your backup first to test for the issue. If the problem persists, try unpairing your watch again using the previous steps, then set it up as new to see if the issue continues.
Cheers.