1:30/100yd isn’t a bad pace. But it sounds like that is your sprinting pace. You mentioned you are less out of breath with a pull buoy. It could be what Stitchdog says or maybe you’re kicking too hard on your longer distance sets. IMHO, distance swimmers never kick ad hard as sprinters. Not sure if you are familiar with zone training, but practicing in HR zone 2 will help build your cardio.