Hi Abu,
From my point of view, to get rid of the first 1 or 2 kilos is not difficult. We take the basic Speed as example, you just need to get proper saddle/ seatpost/ pedals/ tires/ hollow crankset to replace the old ones, and remove the unnecessary accesories, it can be easily 2 kilos lighter.
But you are right, the lighter, the more difficult and costly it becomes. In the end you take account of every gram.
My Archer, as shown, finally has only the frame kept from the original bike. Every single other piece is upgraded, even all the screws.
You may have to use carbon and titanium components, and some special lightweighted parts. For reference, the brake lever here you see weighs only 32g. The front carbon fork only 3xx g. The front wheelset plus tire only 6xx g......

Perhaps I am lucky, because in China these stuff can be found easily, and also because I have a bunch of folding-bike-enthusiast friends to share experiences, just like here.