That's Liberty Pass for you. Once you use all your priority data you get slowed on peak hours 5pm-12am. During the daytime mornings and early afternoons you will see better speeds. You can always buy more priority data or change plans with more data at any time
Liberty Pass is freaking sweet too. I haven't been on it in a while thanks to the late night free zone, but when I used to use up my priority data, I would frequently get speeds over 5 mbps during the day. The Liberty Pass is advertised as speeds up to 1 to 5 mbps.