Why is my online store loading slowly on mobile?

[webdevcat]

I'm trying to help a client with their slow-ish website https://www.dp-tools.de. If I use Google page speed for mobile I can see that it takes 7 seconds to be interactive but nothing in the found problems really tells me why it is actually that slow. I also tried chrome lighthouse and couldn't really see all that much.

Is there another way of checking or maybe anyone here sees why it is so slow?