Run 'vmstat 1' in a different terminal on the RPi while running stuff, and paste the first dozen lines? (You can just CTRL-C vmstat after this)
I ran rtl-tcp a while ago on the RPi and it was really pushing the limits of the IO bandwidth and CPU, but there has been a lot of optimisation in the RPi stack kernels etc since. Using a USB network interface would just push the RPi's limited USB IO even harder.
Reducing the samplerate for rtl-tcp might help a bit if you aren't looking at a wideband signal.
David
I've run vmstat 1, but nothing special change when the problem occour. The cpu NEVER goes more than 50%. Lowering the samplerate didn't change anything (except a very little cpu load lowered).
Still investigating...if someone have some ideas to let me try.... :)