What is the speed of this "slowest service?"

The first thing that comes to mind is interference from another radio in the area or poor signal quality.  Do you know what frequency band the provider is using?  2.4 or 5?

Are all the devices in your home wireless or are some wired?  Did you purchase anything new recently that transmits a signal?  I had an old bluetooth transmitter for a television that gave me the exact type of grief you are experiencing.

You say you have a clear line of sight to the tower but do not say how high both radios are.  Is the radio at your house lower and possibly being impacted by growing crops?  I have several installations that see degraded signal quality this time of year.  Corn and trees really suck up a radio signal.