When we had our copper cylinder and boiler replaced after the cylinder sprung a leak and the insurance assessor found that the boiler was close to rusting through, we decided to go for a combi boiler.
The flow rate of hot water was dramatically improved because the taps were now fed from mains-pressure water (via the boiler) rather than a tank only a
15 feet above the taps. But it still takes a fair time for water to run warm, not because the boiler takes time to heat the water (which it doesn't, because we normally have it on pre-heat) but because the pipe run is so long. For some reason, the original CH/HW system (with the cylinder) had used 15 mm piping for the hot taps rather than 22 mm which is normal for low-pressure tank-fed hot water. Consequently the hot water ran very slowly with the old system, making it very difficult to balance hot and cold water in one of the showers (the other one had a pump in both cold and hot pipes).