Plug a high-resolution monitor into a Mac, open the display settingssrc, and pick anything other than the two resolutions it offers you by default.
Using a scaled resolution may affect performance.
Then nothing visibly breaks. Windows drag normally, the fans stay quiet, and the text is exactly as sharp as it was on either of the two settings that came without a warning. So what is it warning about?
It is worth being exact about what that menu is offering, because the whole article turns on which of these choices costs you something. The numbers below are this monitor's; yours will differ, and the labels underneath say what each one is in general.
Why this monitor is awkward for macOS
macOS lays out its interface for two pixel densities and no others. Roughly 109 PPI, which is a 27-inch 1440p panel, and roughly 218 PPI, which is Apple's own Retina displayssrc. At either one, a menu bar and a body font come out the physical size they were designed to be.
A 39-inch 5120 × 2160 ultrawide is 143 PPI, which is neither. Run it unscaled and the interface is drawn for a 109 PPI world on a panel with a third more pixels per inch, so everything is too small to read. Take the clean halving and it is drawn for 218 PPI on a 143 PPI panel, so everything is too big and the ultrawide desktop you paid for is mostly menu. The readable layout is somewhere in between, and everything in between triggers the warning.
What picking a fractional scale actually does
macOS never resizes your text to fit an awkward layout. It draws the whole screen at twice the layout you asked forsrc, then shrinks the finished picture down to what the cable can carrysrc.
Both of those sizes fall out of the one you picked. Doubling 3200 × 1350 gives a canvas of 6400 × 2700, and the panel is 5120 × 2160. So the shrink is 6400 ÷ 5120, which is 1.25 — and 2700 ÷ 2160 is also 1.25. The same factor on both axes is what keeps the picture in proportion; a scale that came out uneven would squash it.
You can read the middle step straight out of System Report, under Graphics/Displays, while running at 3200 × 1350.
Displays: LG ULTRAGEAR+: Resolution: 6400 x 2700 UI Looks like: 3200 x 1350 @ 100.00Hz
That first number is the tell. The panel's hardware ceiling is 5120 pixels across, and macOS is reporting a rendering resolution of 6400 — a canvas 25% wider than anything the monitor can display, sitting in unified memory purely to be reduced.
Why the text stays sharp
This is the part that resolves the apparent contradiction. Your fonts are never rendered at 3200 × 1350 and then stretched. They are rendered into a 6400-pixel-wide canvas, at full vector quality, and what gets shrunk is the completed image — so every glyph arrives with more source detail than the panel can resolve, and the downscale throws the surplus away rather than inventing anything.
Sharpness was never what the warning was about. The cost is that the shrink has to happen again for every frame that changes.
What the default setting turns off
On this monitor the default is 2560 × 1080. On yours it will be a different number, but it is always the same rule: the default is the native resolution halved on each axis. A 3840 × 2160 display defaults to 1920 × 1080, a 5120 × 2880 one to 2560 × 1440. Switch to it and the same report reads differently.
Displays: LG ULTRAGEAR+: Resolution: 5120 x 2160 (Ultra-wide 5K) UI Looks like: 2560 x 1080 @ 100.00Hz
The oversized canvas is gone, and the halving is exactly why. Double the default back and you land on the panel's own resolution — 2560 doubled is 5120, 1080 doubled is 2160 — so the canvas macOS wants to draw and the screen it has to fill are the same size, and there is nothing left to reconcile. That is true of every monitor, which is why this setting never carries the warning.
That halving decides something much smaller, too: where a single pixel on the panel gets its colour from.
Copying is the operation that disappears. At a scaled resolution the GPU has to work out a blend for every pixel it emits; at the default it reads one value and writes it out. Widen that from a single row to both dimensions and one layout pixel is landing on exactly four panel pixels, every boundary on a real pixel edge. That is the whole difference between the two cases: not that a scaled resolution is expensive, but that an exact one is free.
Seeing the canvas for yourself
You do not have to take System Report's word for it. Set a scaled resolution, press Cmd+Shift+3, then select the file and press Cmd+I. The Dimensions field reports the frame buffer the screenshot was taken from.
At a scaled resolution a full-screen capture comes out larger than your monitor — 6400 × 2700 here — because the canvas is what was captured. At the default it matches the panel exactly, because by then there is no canvas left to capture.
What it actually costs
Open Activity Monitor, then Window, then GPU Historysrc, and drag a window around at a scaled resolution. The shape of the graph is the answer.
The load is a spike, not a floor. On a static screen the last frame is already composited and nothing needs redrawing, so the GPU sits near idle; it only works while something on screen is moving. And on Apple silicon those spikes barely register before flattening out, because reducing a 6400-pixel frame is a small job for a memory system built to move far more than that.
The warning is a survivor from a period when it was not. On an integrated Intel GPU sharing system memory over a narrow bus, resampling every changed frame was a real tax and the caution was earned. That is no longer the hardware most of these machines have.
So the honest version of the warning is much narrower than the one macOS shows you. If you are on a base-model machine driving several displays, editing high-framerate video, or chasing the last of a stutter, the default genuinely hands you back work the GPU was doing. Otherwise what it is protecting you from is a fraction of a GPU you are not using, and what it is costing you is a workspace you can comfortably read.