FBX Inspector Speed + Mask Definition

If you create a model > rig it > animate it > export the FBX > bring it into unity > select it and go to the inspector tab: things are fine.

However, if you create a "Mask > definition > create from this model" to include extra bones, the inspector's speed gets crushingly slow. Especially if you do this to a dozen or so animations.

It takes several seconds for a word to be typed or for a mouse click / drag to register. It took me forever to even realize this was the reason for the slow down. The issue happened gradually because of how I was adding to development. I thought it was something in my importer settings, so I created a new project, imported the same fbx and loaded all of its animations. The engine slowdown didn't begin until I started adding to the inspector tab > selecting the animation button at the top > Mask carrot > definition > create from this model.

Does anybody know what I can do to get the inspector speed to be normal again? The unique mask includes only a couple more bones, which I use as important reference points on my animations for gameplay. The model itself isn't very complicated, and runs just fine during gameplay. It is not at all the case in the inspector for the fbx.

I've submitted a bug report with an example file attached: https://fogbugz.unity3d.com/default.asp?1193351_nporcoukqruep6k9

But it has been a few weeks and, like with my other bug reports, there has been no response. So I'm hoping somebody here can assist or at least tell me I'm not oblivious to something obvious.

I am using unity 2019.2.8f1