So, I checked both monitors.
The LG one will give you a performance increase, when it comes to FPS. The visuals will be slightly worse then the Dell one, because the Dell monitor got a higher resolution. But, you will have less FPS on the Dell one, caused by the resolution.
The FPS increase when using the LG will depend on the game, but you possibly reach up to +15 FPS more with the lower resolution monitor.