It doesn't work that way,
@Wintermist.
@Prosthetics post above is a much more accurate way of saying what I was saying. A mouse cursor moves according to "Dots Per Inch" (DPI), which is a ratio of how many pixels on the screen will be taken into account by the laser as it moves over a mousepad. High-end gaming mice use upwards of 8,000+ DPI, while a standard off-the-shelf mouse uses 500 or so. The higher the DPI, the more accurate the precision of fine movements, the "slower" the cursor will appear to move.
When you increase your resolution, it's like increasing the size of the "playing field". 1080p is a tennis court. 4K is a football field. More pixels = more detail = slower overall mouse cursor movement. Decrease DPI to increase overall mouse speed.
When you downscale, the screen is squeezing all of the detail of a football field into a space the size of a tennis court. This creates an additional job of work for the GPU as it needs to draw the game at about 4x the monitor's resolution, take a picture, shrink it down, clean up the image, then paint it on your screen. Imagine what this will do to the calculations that a mouse needs to draw its cursor accurately on the screen -- it's trying to account for pixels that don't actually exist on your screen-space image, then round things off to come as close as possible to the equivalent position on the pixels that do exist.
This is going to slow down your mouse performance. The devs can't do antything about this -- it's your chosen hardware and your chosen settings.
The only other thing I can think of is (if you have a gaming mouse) ensure you're in Hardware Mode and use the mouse's on-the-fly sensitivity settings (usually a couple of buttons right above or below the wheel) until it's more responsive.
