Blender Modeling High Mesh

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Hm... I've never worked in Blender but I know from other programs that displaying the wireframe on a high res mesh is very bad for fps. Since you're sculpting I would assume you have it turned off already, but well, just in case you haven't, it might help.
Also, in 3D programs there's often an option somewhere that tells the program how much memory it's allowed to use. If this is set too low it will run more sluggish.
There might also be an option (Mudbox has this for example) that tells it where on your hard drive it can store temporal files. If the assigned drive has very little free space this can also impact performance.

What the hell... 20 million polygons? I didn't even know that was possible... but, those that work perfectly, do they have a f**king cool computer or they have a normal one? Are they professionals?

Because if you are a normal dude with a normal computer, is logical that your computer (not the user) says "I can't handle this".

Oh, it greatly depends on which program you're displaying the 20 million polygons in. ZBrush for example has a unique way of calculating them, which allows for high fps on an average computer, even with tons of polygons on screen. 3ds Max on the other hand can make your PC explode if you try loading the same mesh into it