It’s done very fast, normally the workflow is to use rects to trace out the highlight areas, line them up on the main reference points then in the path menu use object to path. Then skew the points as needed, merge paths and add nodes as needed. If you use rects then resize them, the resizing is a matrix transform that scales the strokes, and when you convert to path it normalises back to non transform sizing.
I dont rly see matrix transforms? or i dont understand what you mean
Are you saying here that you do usualy do convert them to paths afterall? why 
rects are good
If you define a rect of 100x100, then set the stroke to 4px, then use the handles to reduce the rect to 100x50, the vertical strokes will be 4px and the horizontal 2px. Convert that rect to a stroke and the path has a constant stroke.
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weird, seems like an inkspace limitation
Entirely inkscape, just a quirk of how they make things easy for the user.
This seems to be a universal behavior of CAD programs. Their SVG export is always this bare minimum quasi-rasterization, not anything more semantic to SVG. They get to check the box on a feature checklist that says "SVG export", folks like the OP get to be continually surprised by how terrible the exports are, and folks like @David_Stone get to prove their chops by knowing how to deal with this 
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