When using obs I get some lag but I would just suggest looking at Nvidia control pannel settings. It was weird for me sometimes. I have a GTX860M also.

I have no experience with SSDs so can't tell. But with the common HDD I've never experienced lag/stuttering during a run.

I feel like I'm necroing but I still feel the need to address this. I play the game on an SSD and the game runs flawlessly, it seems that your lag is coming from either your CPU or GPU having to cache things before being able to run everything at full speed.