Failed to generate a custom world

I (sometimes) failed to generate a custom world.

I was trying out the new worl generation feature (Customized) and played with the different options to decide how my wold will look like. When I thougt it was fine hit the button "Create New World" and waited until my computer finished calculating the world. But instead of a few seconds
(or minutes to be pessimistic) is the pricess after over 45 minutes still running (builting terrain).
I tried to give Java more Ram (6GB [I Have 32] ) ad changed the process priority to "high". But it only sometimes works.

Steps to Reproduce:
I have no plan...
but-->
1. I avoided to use random settings
2. maybe there are two variables set which cause opposite functions (iron generation high min 80; max70)
3. eventually the lapislazuli generation with such settings: center 16; spread 19 --> lowest layer = -3 ??? [no, that's not the problem, I just tried it with everything else on default; but also an issue (just
aesthetically) ]