Previous research has suggested the existence of a general mental speed factor independent from general mental ability. However inconsistent empirical evidence suggests that the speed-ability relation might be more complicated than what was believed. Adopting the joint item response-response time modeling approach developed by van der Linden and colleagues (2006, 2007, 2009), the current study investigates the psychometric properties of the general speed factor and its relation to g in a reasoning task. Personality trait effects are examined as well to account for the speed and ability variances. In line with the earlier findings, results in the current study suggest that the reasoning speed and ability correlation is minimal and explanations for the mixed research findings are discussed within the context of individual differences and test situations.