When your computer records 1080 x 240 fps, most likely it is fully utilizing its resources.
OBS is a program, very demanding the resources. This means, that a substantial part of computer resources must be freed up for normal operation of the OBS Studio.
This can be done by significantly reducing an unimportant parameters, for example, fps.
I don’t understand why you need 240 fps. It is considered, that the difference between 30 and 60 fps is not very large, and you can hardly distinguish between 60 and 120 fps. Reducing fps from 240 to 60 means 4 times reduces the load on the processor.
Reducing the fps and/or resolution is one of the most typical ways to free resources for increasing the frame rate in OBS and Vegas.
80M bps in .264 codec is hard load for the disk system, if you do not have an "SSD only".
The difference between 20, 40 and 80 MBs for 1080p is considered indistinguishable even for a specialist, in any case.
If your final product is intended for broadcast on the web, keep in mind that regardless of the source quality on your computer, the servers delivering the signal to your viewers will recompress your video to 10-12 Mbs.