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Both systems rely on the mains pressure to push the water out of the tap. The only difference is that the unvented cylinder acts as source of hot water in the path between the mains input and the tap, so the water does not have to be heated instantaneously.
If you have five litres/min from the mains, you will get five litres/min coming out of the tap irrespective of which system you have.
Pressure and flow rate are not the same thing.
But if you have 50L/min from the mains you will only get a maximum of around 15/16L/min (in Summer, in ideal conditions) from the best combi boilers on the market. The unvented cylinder will give you 50L/min. That is why mains pressure and flow rate must be checked before fitting either of these products. Dynamic pressure thus also tends to be far better on unvented systems compared to combi boiler systems.
With a combi boiler the hot water to the whole house should ideally be supplied in 15mm pipework. This massively limits the flow rate to the taps as well and causes much larger pressure drops through the system than an unvented setup which will be run in 22mm for significant chunks of the pipe run.
I know you don't directly infer it but lets not try and compare combi boilers to unvented cylinders. An unvented cylinder is a far, far superior solution in terms of hot water performance (assuming the mains is good enough to justify fitting one).
Of course it comes with a far, far superior cost too compared to a combi boiler!
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