I am also experiencing this gradual framerate loss since Dawntrail.
As others have said, the issue arises from being in dense areas like cities such as Tuliyollal/Solution 9 for an extended period of time, usually a few hours (between 1-3). Restarting the game or logging out and back in resolves it.

I play on Steam version.

1. FFXIV, Discord, Google Chrome
2. I have only one character
3. No overheating, CPU is between 60~70C, GPU is between 45~60C
4. No changes to drivers made
5. I run in Borderless Window mode.

Intel i7-8700k CPU
Nvidia RTX 4070 Ti
16GB RAM@3200 Mhz
Samsung EVO SSD
1920 x 1080 Resolution