During laser sintering (LS), more than 90% of the polyamide 12 (PA12) powder remains un-sintered, acting as a support material. The un-sintered powder is known to have a limited recyclability as it remains for a long time in the build at a high build temperature (175 °C), which affects the chemical and physical properties due to thermal impact. Furthermore, the sintered PA12 is subjected to multiple heating and cooling cycles, changing its crystallinity and mechanical properties as well. To date, the changing crystallization kinetics of PA12 during LS are ascribed to post-condensation. This effect results in an increase in the molecular weight, leading to a higher viscosity and thus a reduced coalescence and solidification rate. However, the altered crystallization kinetics of PA12 are hypothesized to originate not only from post-condensation, but also from residual self-nuclei and the degree of polymer entanglements. The aim of this study is to characterize the influence of the thermal history on the crystallization kinetics in PA12 at high heating and cooling rates, comparable to real-life rates observed in LS, using fast scanning calorimetry. The occurrence of the three memory effects, named post-condensation (1), self-nucleation (2) and the competition between dis- and re-entangling (3) and their influence on the crystallization behavior of PA12 is analyzed. At low rates, the results show that memory effects can induce a crystallization temperature decrease as large as 30 °C in PA12. Self-nucleation and disentangling are confirmed to promote crystallization at a higher temperature, while post-condensation and re-entangling obstruct the onset of crystallization. At high rates, the three effects also appear to have a significant influence on the crystallization, leading to an even more pronounced change in the crystallization temperature. In conclusion, the three memory effects are present in PA12 during LS, but their dominance depends on the thermal history.