Obsessional slowness (OS) is a controversial clinical construct whose defining feature is a disabling degree of motor slowness. The present study examined OS in individuals with obsessive-compulsive disorder (OCD) (n = 50) and social anxiety disorder (SAD) (n = 30) using the newly developed Obsessional Slowness Scale (OSS) alongside other clinical and psychometric measures. Exploratory factor analysis revealed a three-factor structure underlying OS comprising perfectionism, mental compulsivity, and bradykinetic features. In multivariable regression analyses, both "soft" catatonic signs, as assessed by a catatonia rating scale, and perfectionism traits, as measured by the Personality Inventory for DSM-5 (PID-5), independently predicted OSS scores. Notably, the categorical diagnosis of OCD did not emerge as an independent predictor of OS severity. Taken together, these findings support the view that OS is a quantifiable and clinically meaningful phenomenon that is not reducible to OCD diagnosis alone. Rather, OS may be more appropriately conceptualized as a transdiagnostic condition within the obsessive-compulsive and related disorders spectrum (OCRDS), exhibiting close phenomenological and dimensional links to the catatonic spectrum.