The mean velocity of blood flow can be calculated if you know the volume flow per sec. and the cross-sectional area. For example, if the volume flow for the vena cava is 100 ml/sec (6 l/min) and the area is 4 square cm. - then the velocity is about 25 cm/sec. The cross-section of the blood vessel is assumed to be a circle, thus the area is equal to pi*r^{2}. This velocity is only about 25% less than the mean value observed in the aorta.

Thus, velocity in this largest vein in the body (Vena Cava) is very substantial - but is anything but sluggish.