GPU activity monitor does not keep track of work being done by the encoder, just the work OBS is doing to render frames. It's also just not a useful way to measure, because it's possible for bottlenecks to exist that degrade performance even when tools indicate less than 100% utilization. When OBS says it can't render a frame, it means it-- it can't render a frame because it can't get access to the GPU. If you're trying to run Rocket League at 1080p with no frame rate cap, and then record it in 240 fps, it is entirely believable that OBS is unable to render and encode frames fast enough in a single PC setup, even on a 2080.
You're saying you were previously recording 1080p240 with a CQP of 16 and went from zero encoding lag to 99% encoding lag without changing anything?
Have a link to a recorded file I could download? Perhaps a short clip?