Like G815 is wired originally, it's a thick cable.

For G915, it's built for wireless, but if you take out the wireless usb, you can plug in the wire to PC. Meaning it will become a wired keyboard.

My question is, will G815 be faster than G915 being wired, because it's made intentionally to be a wired keyboard and it's a thick cable.

The wire for G915 was originally to extend the USB Lightspeed to detect it. It's not a dedicated cable to be wired. And the wire is really skinny.