What is the speed of blood?
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.

References:


Velocity of blood flow in normal human venae cavae.
Regulation of blood flow in single capillaries.