Blood speed isn't a particularly useful metric. We're typically more interested in the volume of blood circulated, as that is a better measurement of blood (and therefore oxygen) delivered to end-organs. Most organs are very good at regulating the speed of flow through their supply, and are more dependent on volume of flow.

One of the more useful measurements we look at is cardiac output (CO). It's just the stroke volume (how many ml of blood is squirted out of the heart every heartbeat) multiplied by the heart rate. This tells you how many ml of blood are sent into circulation every minute. Doppler imaging can be used to show flow speed, though that's much less useful. Measuring CO is relatively easy to measure and is a useful measurement of how much blood is being made available to end organs. The heart rate is very easy to count or calculate, but the stroke volume does require an ultrasound study (preferably a transesophageal study). For an even more useful measurement, you can divide that CO by the person's total body surface area to get the cardiac index (CI) this gives you a more standardized number from person to person, which allows a more meaningful comparison than CO alone.

There isn't a simple answer here since your blood is in all kinds of places with varying pressures and moves to a beat and not continuously, meaning that the speed is different at any given moment. What you can calculate is how long it takes for all of your blood to do a full circulation through your heart.

For a 70kg man, an average heart should have a stroke volume of 90ml (the amount of blood pushed out by a single heart beat) if we divide 5.5L by that we get about 61 beats per full circulation, with a heartbeat of 65bpm this would be about 56 seconds.

So the closest "speed" i can give you is something like 0.94 circulations per minute.

It varies depending on where in the body you measure. It's also function of your heart rate. A heart will pump up to about half a cup per beat, and that will be replaced by the next heartbeat. To transfer that volume to speed, we need to think about the size of the vessel- the aorta is large, so that half a cup doesn't have to move far to make room for the next lot in the next heartbeat. However, counter intuitively, this doesn't mean it moves faster in the smaller vessels. Because there is a greater total cross sectional area across all of the small vessels, it slows down. The water in the big hose moves quick, to supply lots and lots of small hoses which move slow. Searching around tells me it ranges from a fast walking speed in the aorta, to something like the second hand on a watch in some capillaries with a low oxygen requirement.

Search up laser speckle velocimetry or metrology. That's the latest tech.