I got Black Ops 4 as part of a deal when you get an Intel CPU. I got it and in the first few days, it ran perfectly fine, high fps. Then, I started getting low FPS, it went from 150s to 60 and 30 in the lobby.
GPU: GeForce GTX 1080 Ti
i CPU: Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
Memory: 16 GB RAM (15.81 GB RAM usable)
Current resolution: 1920 x 1080, 240Hz
Operating system: Windows 10 Pro
Never use Vsync .. ever in any FPS; those step downs indicate Vsync is On. Most 240 Hz monitors are either G-Sync or Freesync, so if yours is G-Sync (Freesync might work) open the Nvidia Control panel and enable G-Sync and by default it works in full screen mode, so make certain your correct Refresh rate is selected in BO4 and Full Screen is also selected. You can toggle a "G-Sync" indicator from the Nvidia Control panel that will appear in the top left of the screen, or your monitor might have an indicator.