Hydrothermal conversion is performed on organic feedstocks that include solids, by forming a slurry of the feedstock in water and subjecting the slurry to hydrothermal conversion conditions. The hydrothermal conversion conditions may be sufficient to product a carbonized solid and/or liquefaction products. The size of solids (either or both of feedstock or carbonized solids produced in the process) is reduced by conducting a series of bubble-forming and bubble-collapsing cycles on the slurry.