Some SDRs may load the system differently than others. And it's REALLY difficult to sort that out. SDRs are not really well suited to multi-threading, a usual technique to offload processors. SDRC with it's multiple radios working off one front end leverages this to some extent. But, really, a simple SDR is really a straight through process with maybe two meaningful threads, input through FFT to display and input through DSP to audio. Moving data from one CPU thread to another involves moving contents of caches from one processor's cache to another is time very expensive so multiple threads in an SDR tend to all process one after another sequentially on one thread. I guess I am saying program architecture can make a big difference.

In my case I bet the fact that I have a LOT going on at the same time on this machine. (I have an SDRSharp running continuously listening to KUSC for background music. That would be www.kusc.org. I like my music as full meals rather than short 45 RPM record sides. {^_-}) And I keep a poo-pot load of browser tabs and windows open. And often they eat a chunk of processor, too. I tell myself this is stupid. But  - it's just me.)

{^_^}