Each time I start up the computer again, it feels like it runs slower than the last time I used it. I already tried...

This is a pretty common issue with laptops, though often the issue isn't caused by a bad battery. A common cause for short battery life is old or outdated software. Old software tends to absorb battery life faster than the new updated software, while sometimes causing lower overall computer performance also. Try updating to the latest software and see if that helps your battery issue. If this doesn't fix the issue, you might have to replace the battery. Check out the troubleshooting page for the Asus ROG Strix GL702VM-BHI7N09 for more information. Hope this helps!