I see no more than a few additional fps moving to low details. I didn’t find the increase in performance to be significant, compared to the loss of IQ, so I opted to continue with the best visuals rather than the small increase in framerate (which isn’t all that noticeable with a Gsync monitor (no locked Vsync and no screen tearing)).
I wouldn’t imagine that dropping resolution would have that much more impact on performance than dropping all lighting and detail settings to low.
It is necessary to have sufficient RAM, and if you are seeing severe problems you may have some configuration issue that could stand to be poked at.
I’d also note that “An i5” isn’t very descriptive, and there are some differences between 970 chips and implementations too. I have an MSI board in a full sized system, and I gather than mobile versions of both CPU and GPU are marginally slower. Some i5 are dual core (with hyperthreading), rather than 4 core devices, which might make a difference too.