We discuss the formation of antihydrogen atoms (H) in an ultra-cold positron-antiproton plasma. For positron densities np ³ 108 cm-3 the characteristic formation time of stable H is determined by collisional relaxation of highly excited atoms produced in the process of three-body Thompson recombination. Considering on the mechanisms of "replacement collisions" and "transverse collisional drift" we find a bottleneck in the relaxation kinetics and analyze the physical consequences of this phenomenon.