Cold tap low flow rate - only a trickle until 4/5 turns

[Dutch85]

The cold tap in the bathroom basin now requires 4/5 turns before the flow is acceptable. 1 turn used to suffice. They're an old style tap similar to the picture below. We live in a hard water area if that has any significance. Any idea how this can be fixed?
Thanks in advance for any help.

 

[foxhole]

They could have worn or washer beginning to stick, simple dismantle and reassembly with new washer will fix or reveal any problems.

 

[ETAF]

we live in hard water area , and our Bidet hot water tap did this after just 18 mths, but used very infrequently , I removed the assembly - cleaned the seating inside the tap and washer and put back and it worked ok again, only did this a week ago, so not sure how long it may last like that , but i have since received a spare spindle from the manufacturers under the warranty with the bathroom suppliers , as we have had lots of issues with these particular taps and bath/shower mixer