It sets fields related to every entry in the registry. So it'll take a little bit. But in my tests it takes about 1/10th of a second. So it shouldn't be too bad.
Where did you get the idea that Forge was incompatible with versions past 8u111?
Forge for 1.12.2 and earlier require Java 8 specifically, but should work with any update within Java 8.
Forge for 1.13.2, to my knowledge, only works with Java 8 or 10, not 9.
Judging from the screenshot, you might want to return RenderLayer.TRANSPARENT (can't remember the spelling) instead of CUTOUT.
For more information, please read http://greyminecraftcoder.blogspot.com/2014/12/transparent-blocks-18.html.
This tutorial is for 1.8; however, the concept of block layers remains the same.