For me, the difference between 45 and 60 minutes is a touch over 1% (290 to 287). There's a bit more of a dropoff at 75 and 90 because I've been using Zwift races to get back into shape after getting hit by a car/surgery this spring/summer and I haven't been doing many races much longer than an hour. I need to get back into dedicated training, but it's so fun and easy to just log onto the computer and hop into whatever race is handy and get my daily dose of humility.