<p>High-Q nanophotonic resonators are crucial for many applications in classical and quantum optical processing, communication, and sensing. We achieve ultra-high quality factors via a method previously limited to bulk systems, preparing a highly transparent and strongly dispersive medium within the resonator that causes a reduction in the group velocity and a corresponding increase in the quality factor. We implement this via spectral hole burning in erbium-doped thin-film lithium niobate microring resonators, and show Q-factors enhanced by nearly three orders of magnitude to exceed 10⁸. Additionally, we show dynamic control of the resonances via electro-optic tuning. Finally, we present a theoretical model for our experimentally observed resonator linewidths, which are not well-described by the standard Bloch equations. Our results show a dramatic reduction in the erbium dephasing rate under a strong optical drive, leading to much narrower linewidths than would otherwise be expected given the large circulating intensity in the resonator.</p>