The odd thing is it wasn't showing all memory used or excessive load (1.37)
If I remember correctly, the Pentium IV is a single-core architecture with the option for Hyperthreading, which really is just one core with nearly two pipelines.
Thus, a load > 1 is actually a heavy load.
I would have estimated the load of an RTL-FM reception to be low, but obviously, you're using more than one full core at full speed; that means you're very likely running into dropped samples on the RTL dongle.
Aside from the Pentium IV burning power like a stove, it seems to be too slow for the task at hand. Maybe it's busy with something else? Maybe the inefficient 2000s-era USB host hardware is to blame? Or the inefficient sound card interface? Who knows.
Pentium IV predates a lot of really useful processor instructions for signal processing (it's stuck in the SSE2 era...), so even a NEON-enabled ARM of similar clock speed would often beat it in such loads (e.g. a gen3/gen4 Raspberry Pi executing an ARM64 Debian (not Raspbian, which only uses "old" instructions to stay compatible with the original Raspberry Pi)).