Why does Blender lock up if one starts a task on large objects?

I had subdivided a plan by 100 times and when I tried to apply the “Skin” modifier, Blender locked up. That is, it is showing “Not Responding” and I cannot do anything with the UI including saving the file. If I see the task manager, Blender has been taking one CPU thread 100%.

It is not the only time I have seen things like this. It happened when there are many vertices or edges and I try to apply something.

Why is this so? Why is the UI not responding? Why does it do a lengthy task on the UI thread? And why doesn’t it use all the CPU cores?

Subdived a 100 times !!!??? this means 2**100 i mean 2**24 are more the 16 million…!!!
Edit: and you system setup is…

No, not that I executed it 100 times, but in the edit mode, I chose subdivide and set the number of division to 100, creating 10,000 faces.

Anyway, I have tested it and even with 10 division, the skin modifier was slow. With 30, it because unresponsive for several seconds.

I gave up this method and chose a different method.

…and that method was?