Walking speed is shown to be an important indicator of the health status and function in older adults and part of the comprehensive geriatric assessment in clinical practice. The present study aimed to assess the influence of different assessment methods on walking speed and its association with the key aspects of poor health status, i.e., the presence of low cognitive performance and cardiopulmonary disease. In 288 community-dwelling elderly (mean age 82.2 ± 7.1 years) referred to a geriatric outpatient clinic, walking speed was assessed with the 4-m, 10-m, and 6-min walking tests. The mean walking speed assessed with the 10-m walking test was higher compared to the 4-m and 6-min walking tests (mean difference (95% CI) 0.11 m/s (0.10; 0.13) and 0.08 m/s (0.04; 0.13), respectively). No significant difference was found in the walking speed assessed with the 4-m compared to the 6-min walking test (mean difference (95% CI) -0.03 m/s (-0.08; 0.03)). ICCs showed excellent agreement of the 4-m with the 10-m walking test and fair to good agreement of the 6-min with the 4-m as well as 10-m walking test. The presence of low cognitive performance was negatively associated with walking speed, with the highest effect size for the 4-m walking test. The presence of cardiopulmonary disease was negatively associated with walking speed as well, with the highest effect size for the 6-min walking test. In conclusion, in the clinically relevant population of elderly outpatients, walking speed and its interpretation depends on the assessment method, which therefore cannot be used interchangeably in clinical practice.