Levels should not be crashing; it sounds like something specific to certain images. If you can upload a sample image that crashes, I can look into a fix. Please email email@example.com and I will send you the URL (I don’t want to publish the URL here on a public-facing website).
Editing slows down – I’m not sure what you are seeing, so it’s hard for me to determine what the issue is. I have not experience something similar; slider lag usually indicates the GPU is being overdriven, but I would think your machine would be plenty capable enough.
I’ll make a note of the histogram issue. That’s a new one for me.
RAW Brick. The sliders provides are determined by Apple based on the format of the file. Depending on the version of DNG you used, you will get different sliders. The preset could have a bug based on that series of steps.
Yes, the crop and straighten tool needs refinement. I’m not happy with it either 🙁