I had it on the beach with full sun connected to a smartphone and from 13% in 15 minutes it went to 12%.

I don't know what it's made to fix

Amazing construction for its price (7 euros). Very compact, with a stand to stand on, long cable, multiple cables, etc. but...
It is very weak, which I think we should have expected with 3.8W.
It does not charge my XIAOMI Mi 10T Pro in full sunlight.
It charges my Bluetooth speaker normally, but with patience.
Everyone should judge for the application they want it for. It works, though, and it is of good construction.
I am talking about the GD-10X because at first I received a different model which was clearly inferior.

Disappointment!
I bought it before my vacation in Gavdos to charge my phone. Unfortunately, it didn't do anything... I connected an iPhone and a Samsung, none of them showed any signs of charging! The only thing it initially charged was a small 2600mAh power bank, which broke a few days later and no matter how much I let it charge afterwards, it didn't show any power on the phone. I wouldn't recommend it to anyone.