  We present local optical measurements of thermal diffusivity in the $ab$
plane of underdoped YBCO crystals. We find that the diffusivity anisotropy is
comparable to reported values of the electrical resistivity anisotropy,
suggesting that the anisotropies have the same origin. The anisotropy drops
sharply below the charge order transition. We interpret our results through a
strong electron-phonon scattering picture and find that both electronic and
phononic contributions to the diffusivity saturate a proposed bound. Our
results suggest that neither well-defined electron nor phonon quasiparticles
are present in this material.
