Much of the data reported by the Prime Infrastructure API and GUI is cached network data. The frequency that the data is updated is determined by the configuration of background jobs in the system, so yes, you can increase the frequency. You can consult the Admin Guide for more on the list of jobs, and what their defaults are.
Most of the data collected from WLCs is via SNMP, but it really depends on the implementation of the device package, which varies with each version of the device package, version of device software, and device model. So while I can't say conclusively, it most likely is being collected via SNMP.