  In the mixed state of type II superconductors penetrated by an external
magnetic field in the form of a lattice of Abrikosov vortices the dc resistance
is known to increase with increasing velocity of the vortex lattice.
Accordingly, vortex pinning sites impeding the vortex motion are widely used to
preserve the low-dissipative response of the system. Here, while subjecting
superconducting Nb films with nanogrooves to a combination of dc and ac current
stimuli and tuning the number of mobile and pinned vortices by varying the
magnetic field around the so-called matching values, we observe a completely
opposite effect. Namely, the vortex-related microwave excess loss for mobile
vortices becomes smaller than for pinned vortices in a certain range of power
levels at ac current frequencies above 100\,MHz. While a theoretical
description of the observed effect is yet to be elaborated we interpret our
findings in terms of a competition of the effective cooling of the system by
the quasiparticles leaving the vortex cores with the conventional Joule heating
caused by the current flow.
