I agree with your sentiment but you also need to remember that each system setup is different. It could very well be your gauge is inaccurate at that low of a setting. Just saying we don’t live in a “perfect world” setting.
8psi is awfully high. Lower it to 5psi and see what happens. It won’t be that much slower.
As someone else mentioned chill the bottles. I actually put them in 12pks and freeze them. This way you can take 12 out and fill them while the remainder are still chilled.
You are using the long line for liquid, right? Make sure you chill the line too before using. Also, make sure the tubing isn’t getting kinked or anything during filling.
Hopefully you can get this working correctly. I can understand the frustration with paying for something that doesn’t work.