You are referring to boat speed (i.e., knots) , right, not chart refresh speed?
Are you receiving sufficient number of satellites of useful height and separation for accurate sequential positioning and derived speed calculations?
Is it displaying the correct date and local time?
Is it exactly 1/2 actual speed and is that the case all the time now wherever you boat?
Could your Garmin possibly be in Simulator Mode , with user-settable speed?(p.13.)
If not , are you referring to Speed Over Ground at the top, or Velocity Made Good (toward a waypoint ) at the bottom ?
2 different animals, as VMG would decrease if you are not navigating on a COG directly toward the WP, and could easily read 1/2 of SOG.
If none of the above you might check with Garmin to see if they had a SW update to address your issue.
