The default behaviour in Metview is to plot scattered point data as numbers. As text rendering can be slow, the Display Window can become unresponsive when plotting large numbers (e.g. more than 100,000) in this way. The preferred way to plot such data is to use a modified Symbol Plotting icon with Symbol Type set to Marker. This icon can be set as the default Symbol Plotting; alternatively, to use it just for the current plot, you will need to first visualise a Geographic View icon, then drop your data plus the Symbol Plotting icon into the resulting Display Window. See NetCDF Marine Observations Example for an example of an appropriate Symbol Plotting icon.