If possible, don't use x264. It generates a lot of CPU load. For realtime recording a hardware encoder is very useful, like NVENC, which OBS do support.

Also 3500 kbps is to low for 1080p recording. I've tried alot and for high quality recording I do use 30000 kbps in the first step. After post processing I do recode it with 2pass hig hquality encoding to around 12000 kbps before uploading.

A quick example of OBS recording with NVENC available in 720p60 and 1080p60: https://www.youtube.com/watch?v=_NmA1sRu70U