Suppose cyclists are trapped in the bike lane, by curbs or parked car buffers on both sides. Then the fast, slow and average cyclists will have a harder time co-existing.
Mostly a non-issue if designed properly. Cycletracks and paths in The Netherlands are generally 3-4m wide (10-13 feet) which easily accommodates 3 to 4 'lanes' of bicycle traffic. They also generally have proper curve radiuses and no impediments or furniture. Perhaps most important, people there, just like drivers in most of the world outside of the U.S., stay right except to pass or if riding side-by-side are fairly quick to move right. I've had a number of instances where I've been in a hurry to get somewhere and even during rush times it wasn't a problem.