If you’re dipping to 400 now after switching to the 540Hz monitor and uncapping FPS, it could be a few things:
• Recent GPU drivers might have introduced performance regressions. Trying a driver from Aug–Sept 2023 (when it worked) is a good idea.
• G-Sync / VRR might be active on the new monitor — that can introduce frametime inconsistencies. Try disabling G-Sync and V-Sync entirely for testing.
• Check Windows settings: disable Xbox Game Bar, Game DVR, and any overlays (GeForce, Discord, etc.).
• Use High Performance power plan in Windows and check your BIOS for settings like Global C-State Control (disable it).
• In NVIDIA Control Panel, set Low Latency Mode to Ultra and Power Management Mode to Prefer maximum performance.
Also monitor your CPU/GPU clocks with HWiNFO64 to make sure you’re not being downclocked under load. Your system should still be more than capable of 500+ FPS under the right conditions.