I own a 2019 Fiesta Diesel 1.5 TDCi which is around 2 years 10 months old and has a total mileage of 30201 kms (18766 miles approx). Of late operating the clutch pedal is causing me discomfort in the knee in bumper to bumper traffic, and my friend's car (which is of the same make and model) is a little smoother while his car has the same mileage as mine. I called the workshop; they tell me the clutch needs to be checked and others tell me changing the fluid will bring the smoothness back. I don't know why my car's clutch pedal is giving me issues so early?
The theory behind the fluid change is that as hydraulic fluid ages, is exposed to heat and moisture it becomes more viscous, i.e. thicker. Thicker fluid takes more effort to pump, changing the fluid may make it easier to push the pedal. Changing the fluid will also purge any air in the system, which can also make a clutch stiff, you would likely have inconsistent pedal travel too if you had air in the line.
There are other possible issues, you could have a problem with the clutch itself, the master or slave cylinder or the line between them. All of those are more expensive fixes so starting with the fluid makes sense.