If you are using vector software to create a pattern tile, it's generally difficult to "overload the software" in most cases. It's not impossible, but it's difficult.
Actual pattern tiles use only one instance of artwork and then repeat that instance with data references. That's not the same as creating multiple copies of artwork.
To possibly over simplify things... Only one instance of the artwork is stored, then the application says to itself -- [repeat artwork
A X times with a vertical offset of X and a horizontal offset of Y]. This creates the overall pattern.
How detailed, or in depth you wish to make that base artwork is up to you. You can crate practically anything you wish, especially as vector art. Pattern tiles which rely heavily on raster images can be resource intensive in vector applications (i.e. Illustrator, Inkscape, etc.), but not in raster applications (i.e. Photoshop, Gimp, etc.). So choosing the appropriate application to create a pattern in can be important.
If you aren't creating actual software pattern tiles and attempting to draw a pattern completely by hand, then yes, you can overload rendering in some applications at times. I would suggest you explore pattern tile creation in whatever software you are using. Most applications have such a feature. It will make patterns much easier to create and much less resource intensive.
Regarding the second image in the question...
That's not merely a pattern at work. That appears, to me, to be a raster image where the pattern has been used, then some raster adjustments have been made to alter appearance in a specific manner. Something like that is more easily done in a raster application where you can create a selection then merely adjust levels or thresholds.This also accounts for why it's "fuzzy" overall and not clearly defined. Some blurring in the thinker portions helps push the visual.
For vector software, this would take two patterns -- one thin and one thick. The thin pattern would be applied to the inner shapes, then the thick pattern applied to the outer shapes - or merely the strokes on the shapes as below. (Note that it's not inherently going to be "fuzzy" as in the sample image)...
This is a simple star with a very thick stroke (20pts) in Illustrator. Then the different patterns are applied to the fill and the stroke.
On a background filled with the thin pattern...
As far as I'm aware, it's not possible to alter stroke weights dynamically inside pattern tiles. You need separate patterns if you wish weights to be different overall.