Hi Miller,
feeding audio into [bob~] and then going to zero will increase the CPU load by ca. 6%. Clearing the filter or adding a tiny amount of noise brings the CPU load back to its usual level immediately, so I guess it's a problem with denormals. My Pd load meter won't really show the increase, but it's clearly visibly on Process Explorer.
See my attached patch. Tried with Pd 0.47.1, Lenovo Thinkpad L440, Windows 7.
Christof