Well, if your settings didn’t changed much the number fps but slightly reduced the cpu usage. Showed signals of cpu bottleneck, your spotify and discord having issues and your cpu throttling, means either thermal or you were trying to push too much fps to your cpu handle it.
Considering that your cpu is a 4 core-cpu and 4 threads, without hyperthreading. You would be constrained on intensive cpu tasks when you multitask. That would perform similar to i3-3250 which has 2 cores but 4 threads, having hyperthreading each core has 2 threads.
That would put you on near of bare minimum requirements, only clock and maybe some architecture improvements which often were barely 5-15% at best on those intel chips. Those minimum requirements often are about 720p at 30fps, but maybe you got more because your gpu increased the resolution and your cpu compensated a bit because is slightly faster than their minimum.
When you used 144fps, which shows that d2r isn’t exactly cpu bound software but still requires a bit of power from cpu. I tested on beta on Fx-8350 which reached around 200fps at 1080p(cpu bound scenario) with about 60-70% usage and about 60fps at 4k with around 20-30% cpu usage(gpu bound scenario). Meaning that most likely the sweet spot would be 6-8 threads on high framerates. While currently the r7 5800x uses like 7% running about 60 fps and about 15% at 200fps.
Most likely you have a powerful gpu but not powerful cpu to target 144hz monitor, the more regular games coming for newer consoles, most likely the same pattern will happen more often than not.
As general rule, more resolution less likely to have cpu bottleneck. While with more framerates means more cpu bottlenecks on lower resolutions, while on higher ones you would bottleneck your gpu before even bottlenecks your cpu.
Maybe you could set one core for discord and spotify and allocate 3 cores for d2r and see how would perform. Because you have a k variant maybe your clocks could compensate the 1 fewer core and give you something around 90fps, still I would recommend something around 60fps our your case about 72fps(matching 1/2 of your monitor hz).
When you said throttles back, means you are reaching throttling temp or their peak voltage, which would mean unstable clocks or configuration not well done in that regard.
I hope you can have a better experience with the tip, these days folks often do multitasking while they play, while on the older days often folks would only do one task while they play.