If I might amplify on what pHaestus said, velocity is definitly not constant throughout a whole watercooling system.
If you narrow the channel, velocity increases to maintain the same flow rate. If you widen the cross-section, the flow rate drops. Water does not compress (not enough to be noticable at the macro scale). This is why water is slower through a heater core: it has a huge cross section compared to the tubing.
On the other hand, trying to use a narrow nozzle in the waterblock to increase velocity will only work if you have a very powerful pump. Mag-drive pumps cannot produce the pressure necessary to effectively drive something like that. The pumps that can produce that much pressure are, if I guess correctly, much noisier.
It's all tradeoffs.
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