i'm of mixed opinion on bike path speed limits, but my main opinion is that holy fuck we shouldn't even begin to think about it until "bicycle highways" are utterly fucking universal.
Just like with cars, the #1 thing we should do is physically design the infrastructure to encourage and force people to act safely and comfortably. If people are going too fast on a bike path then it's incredibly unlikely that they're just doing it because they're thrillseekers or whatever, they're doing it because they want to get to their destination quickly and either there's no other route available or the path in question isn't sufficiently discouraging high speeds.
At least fucking start with soft speed limits, put up signs and stuff so people know you're supposed to take it slow in an area!