The velocity of energy circulation associated with the uniform precession mode in a ferrite sphere is studied. Specifically, the effect of the boundary conditions imposed by a concentric conducting spherical cavity is considered for the uniform precession magnetic resonance mode of a yttrium‐iron‐garnet (YIG) sphere. Theoretical analyses show that there is a critical ratio between the radius of the ferrite sphere and the conducting cavity where the energy velocity of the uniform precesion mode approaches zero. Spin wave instabilities, as a result of the nonlinear coupling of the uniform precession mode to spin wave excitations, are brought on if the rf‐energy density inside the ferrite exceeds a specific threshold value. Thus a decrease in the energy velocity is expected to decrease the amount of incident power required for the onset of spin wave instabilities. We report that decreases in threshold power for the Suhl first order spin wave instability have been observed as a function of the cavity radius. The experimental cavity consists of two conducting partial hemispheres, one on each side of the YIG sphere.