It depends on the where you’re flying, but even in a really dense scenario (LAX with pretty high FSLTL setttings, below), I went from a base frame rate of 92 fps with no traffic to 63-65 with FSLTL set to these aggressive settings:
This is on a 5,120x1440 49-inch ultrawidescreen, so pushing a similar number of pixels as 4K.
That’s with DLSS set to DLAA (so no upscaling), Terrain LOD at 250, object detail at 200. Everything on Ultra except Bloom, depth of field, and motion blur set to off.
Again, that’s a worst case scenario. Up at 35,000 feet, chasing an FSLTL airliner in the M-346, I was getting about 130 fps with FSLTL at those settings.
I’m essentially getting about 150% of the frame rate as I was getting with the 10900K before turning on the frame doubling. With the 4090 set to DLAA with the frame interpolation, I haven’t found anywhere where I can’t get a smooth flight, even with FSLTL settings pretty high.