Why "generating preview" of photos in my HD takes forever?

I open the folder and it says Generating Previews. But it takes several minutes and sometimes doesnt even complete if I leave the program to check it later - I have to keep moving the screen to prove to Bridge I really want to see my images.  Even when I am accessing a fast SSD.

It happens in folders with thousands of images in RAW (I mean 40Gb in a folder). Am I not supposed to have a folder with thousands of images? Or it is just about Bridge or computer's settings?