This mod requires Fabric, not Forge.
Note: I am no longer updating this mod. If you're on macOS and have the issue that Minecraft has been rendering at full resolution rather than half resolution on retina displays since 1.13, check out retiNO. Otherwise, UltimateBoomer has forked this project and added more functionality under the name ResolutionControl+
Resolution Control allows you to render Minecraft's 3D portion (i.e. the main game, but not the HUD/GUIs/etc.) at a lower resolution, using nearest-neighbor upsampling. It even has a snazzy settings screen (open by pressing P by default) allowing you to adjust this as you go.
I created this mod mainly because Minecraft 1.13 made it render at full- rather than half-resolution on retina monitors, causing it to run really badly on my computer. Any way of downscaling I tried resulted in linear upsampling (rather than the nearest-neighbor upsampling Minecraft used to have), which looked blurry and terrible. Enter Resolution Control, the answer to your prayers!