The performance rating is based on the quality of client connections to the AP: https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Meraki_Health_Overvie...
A lower number means some clients may not have a great connection. Whether you need to do anything or not really depends on what your end users are seeing. If users have poor connectivity it could be anything, device drivers, AP placement, interference, etc. A site survey could be in order.
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