I guess its not how it should work, wiki says that it just 25% more effective than default generator

Pretty sure you are referencing the 1.12.2 wiki for the 1.19.2 version.
The Ingame wiki says (copy pasted out of it)

The Slag Generator is an advanced Generator, increasing its output to a whooping 60EU/t. On top of that, it randomly outputs scrap as a side product.

Please ignore the Github wiki when reporting 1.19.2 issues.