Cycle lanes tend to be designed for speeds below that which a cyclist of even average ability is capable (otherwise, they would tend to more closely resemble roads).

Sheltered from motorised traffic, they cater to a wider age, experience and ability range, and the fact that they are typically narrower than roads exacerbates problems that arise from mixed-ability/experience users sharing same facility.

Features of cycle lanes[1] tend to put cyclists in closer proximity to pedestrians than they would be on a road, and this creates a lack of physical and psychological separation that further reduces practical speed (and utility). They also create new challenges such as situations[2] requiring a cyclist to observe a 270°-arc on the approach to a junction. In the last example, it doesn't matter if I have right of way, and it doesn't matter if I'm wearing a helmet; all it takes is a motorist not to be paying attention or to misjudge my speed and I could be the victim of a fatal accident, and that's not a risk that I'm willing to take.

[1] https://i.imgur.com/CkAxcE5.jpg [2] https://i.imgur.com/0q9PadV.jpg