All of the above really. Firstly, the mat issue is very common. Had it on mine, moved mat, flattened it, checked it, stuck it down, sorted! But then after a while, got gradually worse....ended up being a slipping clutch, so had master cylinder, DMF and clutch done all at once....changed the car immensely! But, has started to get sticky again recently and won't be the clutch as that only has a few thousand miles on it, so suspect its the third of the culprits which is the sticky pedal issue.....bit of WD40 should do the trick. So, check all of the above first, then make the choice on which you feel you can eliminate....good luck!
P.S......the clutch replacement with all of what I had done isn't cheap....lets hope its not that for you!