Hi !
I think it’s unfortunately a normal situation. You ask to the system to multiply by 3 the 4k resolution. With x2 screen it’s ok for RTX4090 so it’s around 8k resolution but it’s the maximum.
I use a RTX4080 and I run x3 4k TV and I have to set resolution as :
x2 sides TV set both at 1440p + main central view TV set to 2160p downscaled TAA 75% = 1620p
I do not find other tips to get better FPS…