  Single photon sources are an integral part of various quantum technologies,
and solid state quantum emitters at room temperature appear as a promising
implementation. We couple the fluorescence of individual silicon vacancy
centers in nanodiamonds to a tunable optical microcavity to demonstrate a
single photon source with high efficiency, increased emission rate, and
improved spectral purity compared to the intrinsic emitter properties. We use a
fiber-based microcavity with a mode volume as small as $3.4~\lambda^3$ and a
quality factor of $1.9\times 10^4$ and observe an effective Purcell factor of
up to 9.2. We furthermore study modifications of the internal rate dynamics and
propose a rate model that closely agrees with the measurements. We observe
lifetime changes of up to 31%, limited by the finite quantum efficiency of the
emitters studied here. With improved materials, our achieved parameters predict
single photon rates beyond 1 GHz.
