I had my bathroom refurbished a while ago. Ever since, when I have my kitchen taps (downstairs) both hot and cold on fully, then turn off the hot tap, cold comes out much faster. Any clues as to why? I mentioned this to the plumber who did the job, who said he didn't know why and left it at that. Is there anything I can do to resolve this problem? I also don't think the pressure is as good in my bath taps as it used to be. I did have a powerful shower installed.
In some systems, hot water comes from a first-floor tank pressurised by a cold-water header-tank in the loft. This header-tank also supplies the cold water taps. Part of the reason for this is that mains water supply to the house is at a high pressure, the header tank provides a lower and more constant pressure that is more suitable for household taps and appliances.
Normally the cold water tank has separate outlets for each to prevent the sort of problems you describe. If your system instead uses a single low-pressure feed pipe to both cold taps and to the hot-water cylinder, that might explain the symptoms.
Since you say 'tap' and not 'faucet' I suspect you may live in a locale where the above applies.
A better plumber might be able to advise what can be done to improve the situation.
The norm is 1/2" to get better hot water pressure. I advise to change the hot water supply lines and valves coming from the heater to the faucets to 3/4". Also when water is heated, the calcium will concentrate and attach to copper & brass; plastic PEX pipes will help solve some of the calcium problems. Tankless heaters are very susceptible to calcium & mineral buildup within the coils. If you want a problem free flow of water, you can install a flush connection and circulate vinegar once a year through the pipes & valves using a large bucket and an electric water pump, that will need to run for about 30 minutes.