To answer my own question:
The scanning interval of wpad can be set using wpa_cli:
wpa_cli -i <wifi-device> scan_interval <time-in-seconds>
This can be added to a script and triggered to run as soon as the WiFi interface is up using the net hotplug.
The default scan interval in wpad is every 1 second as enforced by this patch - 310-rescan_immediately.patch. In my opinion, this is too frequent and unnecessary unless you are using a portable router and would want it to instantly connect.