ShadowPlay might see 5% on the cpu, no big deal. There's other factors at work. Make sure that stupid Xbox DVR is off, that'll kill fps in many games such as cs:go.
Dropping graphics is a 2-edged sword. The game code is all the same, so the cpu is responsible for the same stuff at base, no matter what the settings. The settings themselves are different. The gpu is all about resolution, at the resolution you play at the gpu just has to paint the screen with whatever the cpu sends. If you lower settings across the board the gpu has a much easier workload, so can potentially throw faster fps, which creates higher demand from the cpu, which is already tanked, so no real fps changes. Instead, try selective settings. Drop things like grass details, viewing distance as those are cpu bound tasks, and raise AA, character details etc as those are gpu settings. In nvidia control panel, change physX from the cpu to the gpu. The intent is to put less stress in the cpu, raising its ability, and adding more stress to the gpu, tempering its ability. Overall result is higher fps.
You have a pre-coffee i5. That's 4 cores. No ifs, ands or butts in some games like BF1, pubg, gta5 it's going to take a hit. Those are 8 thread optimized games and running them on a minimum core cpu (4 threads) is going to hurt max potential.
In nvcp you can also adjust settings such as turning triple buffer off, pre-rendered frames to 1 and other tweaks. Most game reddit type wensites/forums have game tweaks settings/patches or at least hints as to what should be set at.
Make sure 2x things are current. Motherboard drivers (especially audio and Lan) and bios. Win10CE can get seriously buggy with some legacy drivers, causing fps loss.