Using monitors at non-native resolution

You may have been hoping I stay out of your thread... if that's the case I apologize.

Of course, gaming at a lower resolution will increase performance, regardless of what the monitors resolution is, because the less pixels your GPU has to render, the more frames per second, it can achieve...which I'm sure is your intent behind gaming at a lower res than 4K.

In your scenario, where all you change is the monitor, FPS will not change as that's strictly a function of the computer. However, monitor lag might increase slightly if you ask the high-res display to scale your low res input to its native res (how much and whether it's even noticable is anyone's guess). However, if you don't care to have black borders around your content, then there should be no penalty (assuming these choices are available in settings).

I don't know how those fast response modes work, so I can't answer that. I have heard that Dell displays with multiple inputs are notoriously laggy from hardcore gamers so if you're considering one of the new Dell's it might be worth waiting for a review.

 

Some additional comments to @VirtualRain's reply:
I've seen claims that seem to indicate that the more inputs a monitor has, the higher the chance that its onboard controller may introduce some lag. This surely depends on a number of factors, so I wouldn't be so bold as to claim this as a general fact, but if you think of it as an extra signal converter that needs to do some specific work, it makes sense that not all hardware and firmware holds the same high quality and specs.

Specifically when it comes to scaling-induced lag, a gamer forum I visited made tests with a number of monitors of different sizes and native resolutions to check this claim, and according to their tests, the numbers were entirely within the margins of error of the test. So at least with screens built later than 2010, you're probably unlikely to suffer reduced speeds from the scaling.

From what I've understood of "fast" or "game" mode, it seems to soften the image. This may indicate that it avoids using the brightest brights and the darkest darks when displaying a picture, so that pixels are less likely to need to change across a significant part of the gamut between white and black between screen refreshes. If so, you're trading color quality and picture detail for refresh speed when using this mode.