4K performance is better than 1080p

Hello, so I was playing MFS2020 and it was unplayable on my 1080p monitor (low fps, around 20-25, and blurry image), so I plugged my PC to a 4K TV. The image was obviously clearer, but surprisingly, the fps were higher (stable 30 fps at 4k ultra vs the 25 variating fps on my 1080p monitor, also at Ultra video settings). Is there I can do to make the game playable in my monitor, it’s not normal I have that issue, also considering I have an RTX 3090, i9 10900k and 64 GB of RAM… .

Thank you for your help.

Be sure to use the correct cable: DisplayPort 1.2 or HDMI 2.1. Also be sure to enable DisplayPort 1.2 or 1.4 compression in the MONITOR SETTINGS.

Thank you for your answer. I use a DP 1.2 cable for the monitor and the settings are correct, this performance issue is only happening with Flight Simulator.

I have noticed similar in other titles in the past, it could be further optimization passes at a particular resolution, or perhaps traffic and or weather variations ?

you bought a 3090 to play in 1080p ??

Your reported 25fps at 1080p is in general not normal, also not in ultra. May be you can share your former (1080p) in-game settings ( resolution, render-scale, etc. ).
Also: in case you have geforce-experiance installed, I realy hope you have disable the Game-Auto-Optimization. And you done no other additional settings in eg. nvidia control.