Hmmmm, interesting. Is there any specific time of day that you access SV? Since MiFi works over the cellular network, cellular traffic through the tower that your location is nearest to will affect your upload/download speeds. When did students in your area go back to school (if there are students in/near your location)? Are you accessing SV near the time when they are going to or coming back from school when they might be using their cell phones? Can you correlate the slowness to a particular time of day? If you know where your nearest cell tower is, might there be a business nearby where employees might be making calls on their breaks? Have there been fires in your area? If your ISP uses microwave to transmit from the cell tower to the central office, smoke will degrade and slow transmission speeds. I don't have any answers; just thinking of the possibilities.
I tend to think that the slowness is a local issue for you. The problem is probably not in the backbone network or in the SV server itself, otherwise more would be complaining about it.