Slow speeds on Jetpack


I have a jetpack for home internet, a MiFi 7730L, and the New Verizon Unlimited plan.  Right after my monthly reset I am able to get max download speeds of up to 1.5Mb/s.  We are in an area that should have full 4G coverage but I rarely get more than 2-3 bars.  After I reach the 15Gb limit for the month I am supposed to have speeds up to 600kb/s but I almost never get over 100kb/s.  Why is this? Does the limiter work by capping the max download speed or does it work by slowing the speed to a percentage of the max available?  If it's the former I should still be able to get the advertised speed.  Any help would be appreciated