Aiming to increase the buffering capacity of too tiny batteries such as the ones incorporated is some ac/dc usb devices (e.g. Luxos U, e-werk usb, etc.) I’m testing the below arrangement.
With a standard e-werk set at 4.9V I repeatedly varied the speed from 10 km/h to 20 km/h alternating in-the-between frequent and prolonged stops (0 km / h) ..
With the GPS switched-on and set at 50% backlight everything seems to work perfectly.

Any comment ??

From my perspective, the problem is not the small buffer battery in the USB-werk or the Luxos U, but their low loading power. You may use the more powerfull E-Werk or the some other dynamo loader.

Your configuration might work as long as the internal electronics of the Luxos or the USB-werk do not have trubles with a voltage input at the plug which is intended as output.

... more powerful ? it doesn't reach 3W at 20 km/h, does it ?
Maybe Dynamo Harvester Plus is more powerful, hopefully!!!

I do not have the E-Werk, but I often read, that it is more powerfull than the USB-Werk.
According to several tests the Dynamo Harvester Plus is also quite strong. Nevertheless, the Forumslader still seems to be the most powerfull loader on the market. There are several versions. The latest version is the strongest, but also most expansive. The older 12V+USB-Version can easily built up by yourself and is much cheaper and not that less powerfull.

There appeared a series of independent tests of several different loaders on the homepage of Forumslader.de and in the independet internet magazin Fahrradzukunft with a series of articles ("Steckdose unterwegs 1-4", translates something like "power plug on the road 1 - 4".) If you have any questions concerning the translation of the test results, just ask. To answer your question: 3 W@20km/h on a 622-weels-bike is about the level which can be expected with the USB-Werk. The Forumslader easily reaches the 3W-level, from lets say already 12km/h. I expect, that the Dynamo Harvester plus also provides 3W already at a speed essentially less than 20km/h.

Thanks for your reply.
Indeed sometimes I have some problems with German but mentioned literature charts are clear enough so far!!
Yes, either Forumslader and Dynamo Harvester Plus seem to be definitely superior when compared to e-werk/usb-werk.
It's curious to see that Zzing at 20 km/h seems to be the least performing.
Perhaps it's true, nevertheless I'm presently using one Zzing that I keep inside the handlebar bag and for supplying/keeping loaded both gps and mobile I think it works fine.
But hopefully one day I'll have the chance to test one Forumslader/Harvester Plus.