The speed of blood may vary from one place to another and may be quite low in the tiniest vessels compared to the biggest one. What is the speed of blood while exiting a normal human heart?
As you note yourself, this depends strongly on the vessel that you are studying. I found this table in reference 1:
It lists speeds between 34 and 45 cm/sec for the inferior vena cava and 12 to 16 cm/sec for the superior vena cava.
For the capillaries I found this table in reference 2, which measured the blood velocity in cats. Nevertheless I think this approach is also useful for an approximation for humans since the capillary system should be comparable:
It lists velocities between 0,2 and 2,7 mm/sec.