Here's what I can tell you from some trial and error I did. A 1 pixel high font looks bad (might as well stick with the Document Overview Bar which SlickEdit already has and close the minimap). 2 pixels can look good when done right. Line drawing doesn't look as good as some sort of font drawing (although it's bright because there's no anti-aliasing). The way to force a 2 pixel font is to do some sampling of say a 3 pixel font and remove 1 pixel. On Windows, you can choose a proportional font for the minimap like "Arial" (not sure about other fixed fonts), and it will be 2 pixels high when you choose a 1 point font size. Not a great solution when the edit window is using a fixed font but just though I'd let you know. Choosing a 1 point font on macOS will give you a 2 pixel high font (on my machine that's what it does).
Originally I thought doing line drawing would be the way to go but it doesn't look as good as I thought. No other products I know of do pure line drawing for a minimap (but they do for document overview bar like stuff). Line drawing won't help speed because the line drawing MUST be done base on some font. There is some additional caching SlickEdit could do but it would cause other performance issues. If it did, then you could cache some font results for say line drawing. The real problem here is that Qt is pretty slow with font calculation stuff. I'd say about 4-10x slower than native code. On Mac, SlickEdit bypasses Qt for edit window font drawing. On Windows, this same optimization is not possible (and we are more proficient with Win32 code) due to how Qt was engineered. There really should'nt be a need for caching anything unless we wanted better support for Softwrap which requires storing the edit window softwrap offsets (unlikely we'll ever bother with that). Note that even though there are WAAAAAAY more extra optimizations for macOS, Windows text drawing is still faster. macOS text drawing graphics is quite slow compared to Windows. Small fonts on macOS look great (better than Windows for sure)! Linux performance is somewhere in the middle but small fonts looks the worst of all the platforms.
Some built-in font generation would be useful. Especially one that takes a 3 point font and reduces it to 2. I suspect if we do that, it might make sense to also support 1 pixel line drawing even though it doesn't really look good.
I hope all this make sense. I've gotten in the weeds a bit.