Choosing an appropriate general purpose HVLP gun for a 5.5 CFM @ 40 PSI Compressor
If your spraygun is too large for your compressor

If you selected a gun that draws more air than your compressor is capable of outputting then you'll be waiting for your compressor to catch up and you will be running your compressor more.

General things to keep in mind


The type of coatings you will be spraying: Different HVLP guns are designed for different types of coatings.  
Nozzle/Tip size: The nozzle is one factor in controlling the spray pattern.  Different size nozzles are used for primers, clear-coats and stains.
Adjustability:  Controls for fan size, fan shape, air volume and material volume.
Setup and configuration: At minimum, you will need a regulator at your spray gun for fine tuning.  Moisture and other contaminants are also concern, so proper routing and filtration of the compressed air will help to reduce contaminants like oil and excess moisture.