The rTCA cycle's widespread conservation across life suggests it represents a crucial evolutionary innovation, yet these computational analyses show it as just one of many possible solutions. The apparent paradox may be resolved by considering previously overlooked kinetic factors. These reactions may be thermodynamically feasible but kinetically constrained, either through extremely high activation energies or through complex multi-step reaction mechanisms. Although many fixation cycles are theoretically possible, their practical viability depends on whether the desired reactions can proceed faster than competing side reactions. A crucial factor is also where metabolites accumulate in these cycles, as these accumulation points create vulnerability to unwanted side reactions Here, we characterize the reaction mechanisms and kinetics of the uncatalyzed rTCA cycle and structurally related reaction networks that share key chemical transformations. The recent development of high-throughput methods for characterizing reaction kinetics now makes it possible to comprehensively investigate reaction networks at a scale that is comparable to the foregoing studies that were limited only to thermodynamic characterizations. In this work, the kinetics of uncatalyzed rTCA cycles and plausible competing side reactions are characterized for the first time using a computational reaction exploration methodology. Kinetic modeling of the resulting network confirmed the feasibility of the primitive rTCA cycle under idealized conditions but also showed that negligible carbon fixation flux results when the kinetics of competing offcycle reactions were accounted for. The major off-cycle fluxes occur primarily at reaction steps that in modern organisms are catalyzed by enzymes. This correlation suggests two possibilities regarding prebiotic carbon fixation. The challenging kinetics at these steps might indicate that an uncatalyzed rTCA cycle was not a viable prebiotic pathway. Alternatively, these kinetically challenging steps might represent the precise locations where early catalysis became essential for enabling a primitive rTCA cycle. These mechanistic analyses identify specific kinetic barriers in the rTCA cycle that would require catalysts for sustained carbon fixation. Such barriers persist despite the previously reported thermodynamic favorability of numerous rTCA analogs, indicating that thermodynamic analysis alone provides insufficient evidence for evaluating prebiotic reaction networks.