How much performance do you lose rendering 1440p on a 4K OLED monitor?

The actual FPS depends on the game and settings, but your RTX 4070 will lose a lot more performance rendering native 4K than rendering 1440p. For competitive games such as CS2, PUBG, and Call of Duty, 1440p rendering on the 4K OLED should still perform close to 1440p-native performance. If text clarity matters several days per week, the 4K panel may be the better compromise: use 4K output with scaling or DLSS when available, and lower the render resolution when you need higher FPS.

CS2 does not support DLSS, so you would need to use its resolution settings or another scaling method there. A 1440p signal on a 4K screen should be playable, but it will not be quite as crisp as native 1440p because 1440p does not map perfectly onto a 4K pixel grid. Try enabling a custom resolution or resolution scaling on your current display first so you can judge the softness yourself.

Rendering at 1440p takes essentially the same GPU power whether the display itself is 1440p or 4K, so simply connecting a 4K monitor will not reduce your FPS. The drawback is image quality: 1440p does not scale evenly to 4K, so the picture can look a little softer than it does on a native 1440p panel. The UI and text may still look sharp if the game renders them at the output resolution, though.

For games that support it, using DLSS Quality or the game's internal resolution scaling is usually preferable. You can keep a 4K output for sharp menus and HUD elements while rendering the 3D scene at a lower resolution. DLSS Quality at 4K is roughly in the 1440p internal-resolution range, but it has some processing overhead, so native 1440p will usually be a little faster. The difference is generally small compared with the performance gap between rendering the full game at 1440p and full native 4K.