Bridges are designed specifically to bypass censorship against Tor (1), and have the known (inherent?) downside of slowing down the connection (2). This is one of the two reasons why you should not use them if you are not facing censorship (the other is that this way you leave more bandwidth available for those who actually need a bridge).
I guess there is a time when Tor is faster because it is used by less people, but I have no idea when it could be. I personally don’t see any time-related variation in speed, but I think this is not relevant. I hope someone else can provide more useful info about this.