Some issues might be that the mifi device is getting overheated. Whenever I have a "slow down" occur, I cool the hotspot down. (Either by putting a cold pack on it or by turning it off for a few minutes.
I have had this mifi since June 2012...Novatel 4510L mifi
Last month I used 19GB. So far this month I'm at 2.81 as of 6/8.
Check Useage :
Check Useage https://members.millenicom.com/members/index.php?fuse=clients&view=Profile_Products
To check your usage, log into the millenicom.com Members area. Click Products. You will see a blue hyperlink with your package number. Click the link. Scroll to the bottom of the page to find the usage field.